Hi Evelin. These types of sentences are called conditional sentences. We use were because it's the subjunctive mood of the verb, but it also depends of the type of conditional that you're using. The "were" is used mostly in unreal conditionals which express things that are hypothetical or improbable. Here you can see a complete explanation with examples. https://www.grammarly.com/blog/conditional-sentences-was-instead-of-were/

If I were you, I would kick her out of the house. (A wishful thought, of being someone or something you're not) If I was there, I could've saved her. (A wishful thought of wanting to change something you can no longer do anything about, or because the moment is gone, it's in the past) Another example, If I were beautiful, I would've been married to a wealthy man. If I wasn't so careless, he wouldn't have ended in the hospital. If I were a bird, I would escape this dungeon. If I was with you last night, things wouldn't have turned out this way.

Thank you both for the explanations.

We can use "were" for real plural past (this is the rule you already know), and for unreal present, both singular and plural: If I were, if she were, if it were, if we were, if they were...

Hello Evelin. These sentences are conditional sentences. Dependent clauses of present unreal conditional sentences use the past tense of all verbs except to be. to be uses were in all persons in these clauses. I hope this help you.