Study materials
Compare two conditions. What differs is not the time but your estimate of the odds.
If I see her, I'll tell her.
If I saw her, I'd tell her.
The second sentence uses a past form although it is about the future. The past tense works here not as a time but as a distance: it pushes the situation away from reality.
The pattern and what it means
The formula is short: if plus the Past Simple, then would plus the base verb. The halves can swap places without changing the meaning.
If I had more time, I would learn Japanese.
I would move abroad if I spoke the language.
This is about now or about the future, not about the past. The first sentence means I have no time at this moment.
Would does not cross into the if
The rule is the same as in a real condition, only the forms differ. The condition sits in the past tense; would lives exclusively in the other half.
If it would stop raining, we'd go out. → If it stopped raining, we'd go out.
In speech would is nearly always contracted to 'd: I'd go, she'd call. It is barely audible, so it is worth listening for.
If I were you
With the verb be this structure traditionally takes were for every person, including I, he and she. The form was does turn up in speech, but were sounds more correct and more neutral.
If I were you, I'd wait a week.
If she were here, she'd know what to do.
The first phrase is the commonest way of giving advice in English. It is softer than should, because you are talking about yourself rather than instructing the other person.
Could and might instead of would
In the second half would can be swapped to change the shade. Could adds ability; might adds uncertainty.
If we left early, we could catch the train.
If you asked him, he might agree.
The mistakes everyone makes
The first is putting would after if.
If I would have time, I'd call. → If I had time, I'd call.
The second is taking a present form where the situation is imaginary.
If I am rich, I would travel. → If I were rich, I would travel.
The third is taking the imaginary form where everything is perfectly real.
If it rained tomorrow, we would stay home. → If it rains tomorrow, we will stay home.
All three are caught by one question. Do I think this is possible, or am I imagining something unlikely? Possible takes the present form; imagined takes the past.
Try it right now
Say in English what you would do if you had a spare free week and no obligations.
Then give somebody advice starting with If I were you. Check that no would has crept in after if.