SYNOPSIS: Sometimes it must be “if I was”, but at other times it can be “if I were” — and for some speakers in those cases, perhaps even must be “if I were” in their idiolect.

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Sentences with the subordinating conjunction if normally contain two clauses, each with its own subject and verb. The question asks what to do about the past-tense be verb in the “if” clause.

Unfortunately, as it’s currently worded the question can have no answer that is simultaneously all of short, complete, and correct. That’s because it doesn’t provide enough context to know which one of many possible cases actually applies here. I must therefore cover them all.

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Classification schemes

David Maule in his 1988 EFL paper titled ‘Sorry, if he comes, I go’: teaching conditionals suggested that English conditionals be broadly classified as one of four types depending on whether their outcomes were real vs. unreal and past vs. non-past. (Maule classifies these based on their “then” part not on their “if” part, and as we shall see, this is a useful way to organize them.)

• Class A: real non-past
• Class B: real past
• Class C: unreal non-past
• Class D: unreal past

Maule discovered that most English conditionals do not fit into the narrow models typically presented to EFL students learning English.

Christian Jones and Daniel Waller built on Maule’s work with their own EFL paper in 2010, If only it were true: the problem with the four conditionals. The authors sampled a random assortment of conditionals from the British National Corpus and classified each as being one of Maule’s four categories listed above. They discovered that the real cases contained patterns in both the past and the non-past that appeared very frequently in real English, but which are rarely taught to learners.

The Reals

The class B “real past” cases fit into three patterns:

1. If + present simple, past continuous
2. If + present simple, past simple
3. If + past simple, past simple

Of those three, the final pattern of having past simple in both clauses was by far the most common of the three. The sample provided for that case was:

... if you wanted^[real] to know the answer ... you had^[real] to keep zapping from channel to channel.

Converting that into the first person singular to align with the asker’s question gives us:

If I wanted^[real] to know the answer, I had^[real] to keep zapping from channel to channel.

And it just one step more to swap out want for be:

If I was^[real] interested in knowing the answer, I had^[real] to keep zapping from channel to channel.

So here we discover the first of what shall prove to be several answers to the asker’s question:

You use If I was in the “if” part when the “then” part is in the simple past.

These are always conditionals from Maule’s class B. It would not be grammatical to use “If I were” there.

These “real past” cases happen all the time in real speech and real writing, as Jones and Waller prove.

Consider this arrangement:

If she was^[real] already home when he got there, then she took^[real] the bus.

That’s a real past case on both sides, and it would be ungrammatical to use “If she were” to attempt to mean the same thing. You can also use a modal perfect in the consequent along with that past simple in the “if” part:

If she was^[real] already home when he got there, then she must have taken the bus.

If she was^[real] already home when he got there, then she will have taken the bus.

Those are all real cases, and you know by the “then” part.

The Unreals

For Class C, the unreal non-pasts, there are many example patterns provided, but the most common case by far uses “if” with past simple or with a modal, then some modal in the consequent.

One provided example there is:

... I’d give it a good hiding if it didn’t behave.

However, there are many other Class C patterns, such as:

... if we could get three or four items, that would be very nice.

... if two members of staff happen to fall in love and decide to marry it would be churlish to be appointing blame.

The thing about using the past simple in something like “If it didn’t” is that without looking further along in the sentence, this alone is not enough to reveal whether it’s a Class B type that will take a real consequent or whether it’s a Class C type that will take an unreal one.

“If only it were true”, “I wish it were true”

Because we use the simple past tense in English for real and unreal conditionals, you normally cannot know whether it’s the unreal case until you hit the “then” portion. But in one unique yet common case, you can, and that is when a singular subject is governing the verb be in the past. That’s because the unreal case uses were no matter whether singular or plural.

So we could say:

If a staff member were to fall in love, it would be churlish to assign blame.

That’s a Class C conditional because the “then” part has a would be in it. But you already knew it was going to be a hypothetical case when you saw the “If a staff member were” in the first half.

Recasting that into the first person singular provides the second answer to the asker’s question:

If I were to fall in love, it would be churlish to blame me for it.

This special, modally marked form of be is used only for an unreal hypothetical. It is a relic of the Old English past subjunctive, and it was once used for far more than we use it today.

Here alone can you detect through the morphology of the verb that it is anything other than the past simple. This is a Class C conditional because it has an unreal non-past in its consequent: “would be churlish”

You cannot go wrong by using were for hypotheticals like this, as it has been the preferred use for centuries, particularly but not exclusively in America. Many careful writers still choose to observe this distinction: you need but read some recent issue of The Economist magazine from the UK to find plenty of examples of this. Indeed, English teachers at American schools have been known to mark various hypothetical uses of was as “wrong”, saying that it “should” be were.

Optional were in Class C conditionals

However, you should not flinch if — nay, when — you hear someone say “If I was... I would...” as a Class C conditional in casual speech. This sometimes happens even in educated speakers and writers, so you should not make anything of it. Some writers prefer not to do that, but unless the person complaining is your English teacher, you shouldn’t let it get to you. (Yes, this is ungrammatical for some people. For others, it is not.)

It could be that those writers or speakers using “If I was...would” in their conditionals have chosen not to convey the nuance, or perhaps did not consider such a distinction meaningful in their own speech. Some are even unaware that the distinction exists.

Because of the redundancy in language where the would in the “then” part gives it away, it’s not really needed anyway; everyone will still know what you mean.

These forms are still unreal cases even when they aren’t modally marked as unreal, singular were. Because in all cases except for this unique case of was/were you cannot ever morphologically distinguish a real case from an unreal one in English, you have to decide whether it’s unreal by looking at the “then” part, not the “if” part (at least, not reliably).

That means you need to train yourself to tell the real case:

If she was^[real] already home when he got there, then she took^[real] the bus.

From the unreal case:

If she were^[unreal] already home when he got there, he wouldn’t have^[unreal] to run pick her up himself.

Even when the unreal case uses the past simple not unreal past in the “if” part the way some speakers do:

If she was^{[“unreal”]} already home when he got there, he wouldn’t have^[unreal] to run pick her up himself.

That last example above is real in form but it is still unreal in sense because of the would. Some writers disapprove of that style of using was for a hypothetical, but it’s not uncommon, especially in speech.

Moreover, you cannot somehow make it be “less hypothetical” merely by using “was...would”; that’s just as hypothetical as “were...would” for the reasons already stated.

One final common construction uses past perfect in the “if” part and a modal perfect in the “then” part:

If she had been^[unreal] already home when he got there, he wouldn’t have had^[unreal] to run pick her up himself.

Although that’s a common way to set up a unreal case with perfects on both sides, there are many other ways, including using a non-perfect unreal past in the “if”:

If she were^[unreal] already home when he got there, he wouldn’t have had^[unreal] to run pick her up himself.

Yes, it’s somehow “unbalanced” with respect to the perfect aspect, but English doesn’t have an obligatory sequence-of-tenses rule like some languages do, and we often use a simple past instead of a perfect one because it’s...simpler that way.

Inverted Conditionals without if

There is one relatively uncommon place where you pretty much do have to use were not was in a conditional, and that is when you use inversion to forgo the word if altogether:

Were^[unreal] there any other way, we would have^[unreal] found it.

That’s the same as saying:

If there were^[unreal] any other way, we would have^[unreal] found it.

or even as saying:

If there had been^[unreal] any other way, we would have^[unreal] found it.

But that last one lends itself to an inverted version:

Had there been^[unreal] any other way, we would have^[unreal] found it.

The subject–verb inversion is something of a stealth conditional because it doesn’t use the word if. The inversion alone is enough to signal that it’s what used to be called a “subjunctive” use (back when English had an actual subjunctive). It doesn’t have to use be, but if you do use be for it, you should certainly use were. Other verbs in the past tense work the same, with the inversion signalling the conditional:

Had^[unreal] they but asked, we would have^[unreal] gladly told them.

You’ll find this “subjunctive inversion” style in formal writing, but very rarely if ever in extemporaneous, casual speaking. That’s because inversion isn’t all that normal, so it’s a marked form. Consider how stiffly formal this Steven Brust quote mentioned in this answer sounds:

To be more precise, and state the matter in its simplest form, we believe that were^[unreal] any of the events in the previous volume of such a nature that they could be omitted without severe damage to the narrative, we should have omitted^[unreal] them to begin with.

        ― The Lord of Castle Black, by Steven Brust

There instead of writing out the conditional the long way with “if any were”, to be more formal Brust wrote it with inversion: “were any”. (He’s also playing on the modal duality of should, but that’s something else again.)

If you ever get the chance to read English literature from a couple centuries ago or better, you might even come upon conditional inversion used with the bare infinitive in what has historically been called a “present subjunctive” use:

Be ye^[unreal] man or mouse, still shall ye say nothing!

That’s using inversion to skip the if, as though it were:

If ye be^[unreal] man or mouse, still shall ye say nothing!

Nobody talks that way anymore, and nobody writes that way anymore, either, not unless they intend to represent the speech of centuries long past. Instead we’d just say:

No matter whether you are a man or a mouse, you still will say nothing!

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Further Reading

I have related answers here:

• subjunctive in a “non-if” hypothetical clause
• “Be them” or “be they”?
• If I were him, I would doubt if she (is/was/were?) serious about this relationship
• How is the English Subjunctive Composed?
• Present subjunctive with synonyms of lest

I'm hesitant to disagree with @Barrie-England given his credentials, but I don't believe "were" would be grammatical in this case. As he mentioned, "it conveys varying degrees of remoteness from factuality." In your example, it's unlikely that you're asking your reader about his or her wishes, opinion, judgement, or possible state. Your statement is asking for clarification of a definite and factual characteristic.

Answering the question with a "were,"

The interviewer asked me if I were fluent in English.

would leave the audience (me at least) wondering what the uncertain event, action, or state would be. Expanding it to something like the following to add the necessary element of uncertainty would seem grammatical to me.

The interviewer asked me if I were fluent in English, would I then travel to England.

That's probably an awful example, but I see "were" being used with words that denote uncertainty: would, if, should, could, wish, must, etc.

I see "was" being used to denote certainty. Presuming your question was expected to be answered with a "Yes" or a "No," "was" should be used.

Really, though, shouldn't they have responded with:

The interviewer asked me if I am fluent in English.