Is this rule "If" specific?

Question

When I was in High School, I read a rule, that was tricky, and I still remember. The rule is:

If something has happened but we wish it happened the other way by using "if", we should change the was/were in supposedly reverse order with respect to the subject. For example:

If I were a bird. (still remember it from high school)

So I am not a bird, but wish if I was a bird. Here was should be replaced by were.

Now what I understand this rule should apply when something has not happened or something we wish but can't happen. This type of expression can also be conveyed without using "If". But I haven't see this rule used without if. For example Thought it were you (But it isn't)

So my question is Is this rule an if specific rule rule?

Also as a side note: I would also like to know if such rule exists in case of present tense i.e. jumbling am/is/are?

Answer 1

were when used in cases like that is the "past subjunctive". The subjunctive is used to refer to things other than actual facts; e.g. counterfactual statements or indirect commands.

If I were the king, I would...

This means I am not king but indicates something I would do in that case.

It also can be stated as

Were I the king

which has the same meaning as the above "If I were the king"

Lastly, the past subjunctive also shows up in statements of wishing

I wish he were here

This statement indicates he is not here, but that I want him to be here.

Thought it were you

This statement is not grammatical. Verb-subject inversion only occur with counterfactual statements. "Thought" needs a stated subject (it's not impersonal and being past tense, it can't be an imperative), but "it" can't be the subject (since inversion of verbs only occurs with subjunctive counterfactual statements). Thought also would only have an indicative clause since thinking indicates how things are (indicative) and now how we want things to be (subjunctive).

You'd have to say something like "I thought it was you"

Answer 2

This question has been thoroughly discussed at ELU.

Briefly speaking, "were" is not a past tense of "to be". Instead, it is subjunctive mood:

The subjunctive mood is used to express a wish or possible situation that is currently not true.

And subjunctive is preferred (other sources say, it is required) to use were, not was.

For example, you may say:

If I were a bird tomorrow.

Here's one more example for better understanding:

If I were wrong, others would let me know.
If I was wrong, please forgive me.

Here, the former sentence is subjunctive: I say that that the first clause is not true (I don't believe I can be wrong). Literally, I say, "others have not let me know, so it is argument for I can't be wrong".

The latter sentence is past tense wrapped into conditional mood: I assume I might be wrong, and, if so, the second clause apply (I plead forgiveness).

An often used example of subjunctive mood is:

If I were you, I would...

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From comments (note "thought", not "though"):

Thought it were you

To me, it does not seem to be subjunctive. Instead, it looks like two linked phrases: {I} Thought {that} it were you. However, there is a bit of ambiguity, as you may also say: Thought it was you.

To my understanding, was versus were can be different due to slight differences between subjects, with a slight change with the word order.

         ┌──↓
Thought it was you

meaning "that object appeared to be you"

         ↓──┐ ↓──┐
Thought it were you

meaning "you were that object I've thought of"

Note that in English, the subject/verb order is strict, but this rule can be violated in colloquial speech.

Update I just thought it may also be an impersonal verb as well:

{I} though {that} (it was (you {doing it}))