Learn English – Can relative pronoun be omitted in the following sentence

The grammaticality of your sentence is a good example of the mess people get into when they starting thinking about grammatical case in English, and why "whom" gives so much trouble.

There is such a rule, but it is not about whether the relative pronoun would be "who" or "whom": it is about whether or not the NP (noun phrase) which is the antecedent of the relative clause is the subject of that clause.

In

The man who came yesterday was French

you cannot omit "who", because "the man" is the subject of the relative clause.

In

The man (who(m)) I saw yesterday was French

you can, because "the man is the object of "I saw".

In simple cases there is only one clause introduced by the relative, so if the antecedent is not the subject of it, it will be in the objective case, and "whom" is appropriate, if we use it at all.

The man (whom) I saw yesterday was French

But the given example is complex, with an embedded clause [I hope [(that) he will come], so the antecedent "the man" is actually the subject of the (inner) embedded clause. Thus the structure is

I have (a friend (who I hope (he will come)))

where the 'he' is replaced by the 'who' (not 'whom') in the clause above, and then optionally omitted because neither it nor its containing clause is the subject of the relative clause.

It's worth noting that this is a case where people who do want to use "whom" sometimes get confused, and use "whom" 'incorrectly'.

There are four factors which decide whether a relative pronoun (or the word that) can be omitted or not:

1. Is it a defining relative clause?
2. Does the main verb in the relative clause have a separate Subject?
3. Is the relative pronoun the first word in the relative phrase? (or is it preceded by another word, for example a preposition)
4. Is the word who, which or that?

If the answer to the questions above is yes then the relative pronoun can be omitted. There are lots of duff websites around which might tell you that you can only omit these words if they represent the Object of the relative clause. This is hogwash. You can omit the pronoun as long as it is not the Subject of the matrix verb in the relative clause. The pronoun can, for example, be omitted if it is the Object of a preposition or Subject of another clause embedded within the relative clause.

Here are some examples to illustrate each point. An asterisk, *, denotes an ungrammatical example.

We cannot usually drop a pronoun from a non-defining relative clause:

• The agent I met up with wants you to phone him. (defining)
• *Your father, I met up with yesterday, wants you to call him. (non-defining)

We cannot drop the relative pronoun if the main verb in the relative clause does not have a separate Subject:

• I don't like the elephant you bit. (matrix verb in r-clause has a Subject, you)
• I don't like the elephant you said bit you. (matrix verb in r-clause has a Subject, you)
• *I don't like the elephant bit you. (verb in r-clause has no Subject)

Notice that in the second example the missing pronoun represents the Subject of the verb bit.

If the pronoun is embedded within another phrase, for example a preposition phrase, then it cannot be omitted:

• That's the circus I work in.
• That's the circus in which I work
• *That's the circus in I work.

We can drop the pronouns who, which and the word that, but we cannot drop the pronoun whose:

• That's the table I bought.
• That's the girl I like.
• That's the girl whose table I like.
• *That's the girl table I like.

The Original Poster's example

"For someone used to the tiny creatures we get in England it was something of a shock."

The word someone here has been post-modified by an adjective phrase. Some people argue that this is the result of removing who is from a relative clause. If you have a defining relative clause which uses the verb BE, you can often drop the relative pronoun and the verb BE. Whether it is now a kind of relative clause, or just an adjective phrase, or participle phrase modifying the noun is up for debate. Here are some more examples:

• The man [who was] going into the chip shop was an undercover agent.
• The elephant [who was] interested in the buns was rather plump.

Here's an example where you can't:

• The people who were blond preferred detective fiction.
• *The people blond preferred detective fiction.

We often cannot do this if what's left of the clause is only one adjective.

The sentence has a real relative clause modifying the phrase tiny creatures:

• ... creatures [which] we get in England

This is a defining relative clause, and the verb get has its own Subject, the word we. There are no other words preceding which. We can therefore happily drop the relative pronoun.

We could rewrite the sentence like this to show where potential words have been omitted:

For someone who was used to the tiny creatures that we get in England it was something of a shock.

Note:

Many grammars use the terms restrictive/non-restrictive or integrated/supplementary to describe what I've called defining and non-defining relative clauses.