Learn English – the difference between ‘Conditionals’ & ‘Subjunctive mood’

I usually don't like to split my answer into two under the same question, but I believe this is a good exception.

To give you some quotes from a movie I like. Here are some parts of Prometheus (2012), which I especially like (no matter what others said about it), enough that I can see these scenes immediately and vividly just by reading these quoes. Also, they can help pointing out why Conditional I, II, and III, while being useful, are insufficient.

Meredith Vickers: Take us home!
Elizabeth Shaw: If we don't stop it, there won't be any home to go back to!

This is a good example of Condition Type I (or, according to PEU¹ 257.2, present tense with future meaning).

Peter Weyland: [as a holographic recording]
Hello, friends. My name is Peter Weyland. I am your employer. I am recording this, 22 June, 2091. And if you're watching it, you have reached your destination. And I am long dead. May I rest in peace.

This structure of "if you're ..., you have reached ..." is a good example of something not Conditional I, II, or III. When I heard it, it was almost as if he should said "..., you would have reached ...", but he didn't. He simply said "..., you have reached ...".

Charlie Holloway: David, why are you wearing a suit, man?
David: I beg your pardon?
Charlie Holloway: You don't breathe, remember? So why wear a suit?
David: I was designed like this because you are more comfortable interacting with your own kind. If I didn't wear a suit, it would defeat the purpose.

This is a good example of Condition Type II (or, according to PEU 258.2, if + past; would + infinitive without to).

Janek: You know, if you wanna get laid, you really don't have to pretend to be interested in the pyramid scan. I mean, you could just say, "Hey, I'm trying to get laid." Heh.
Meredith Vickers: I could. I could say that, right? But then it wouldn't make sense why I would fly myself half a billion miles from every man on Earth if I wanted to get laid, would it?
Janek: Hey, uh, Vickers. Hey, Vickers. I was wondering... Are you a robot?
Meredith Vickers: [scoffs] My room. Ten minutes.

Another good example of something not Conditional I, II, or III. According to PEU 247.1, this "if you wanna ..., you (really) don't have to ...," is categorized as "the same tenses as with other conjunctions".

Elizabeth Shaw: Right. All you do is fly the ship.
Janek: That's right.
Elizabeth Shaw: But you must care about something, captain. If you didn't, why are you here?

Another good example of something not Conditional I, II, or III. I am not even sure which item in PEU it would fall into. Perhaps 262.4, mixed tense: "Sometimes a simple past tense is used with if where a past perfect would be normal. This is more common is American English."

^{1. PEU = Michael Swan's, Practical English Usage.}

English does not have a subjunctive mood, nor a conditional mood.
Despite what you may have been told in school; if you've been misled, I'm sorry about that.

English does have a lot of modal auxiliaries, some of which have meanings (but not grammar)
that reminds scholars of some of the uses of subjunctive and conditional moods in
languages that have them, like Latin or Sanskrit.

But there is no subjunctive, no conditional, and no future tense in English. The constructions that are sometimes given these names are all uses of modal auxiliary verbs, with syntax to match.

This has been explained over and over again, but it's always surprising the first time.

Here are some answers to questions about the so-called "subjunctive" in English.
As you can see, it doesn't work quite the way you have been taught.