A structure used for talking about the real world, and often refers to general truths – things which always happen under certain conditions. The present tense is used in A and B sections:
I like to walk if the weather is warm.
It is commonly used to state scientific facts:
Your immune system responds if bacteria enter your body.
We often use the zero conditional to give instructions; B: imperative form:
Press the button if you want a receipt.
If he calls, tell me.
Often refers to the future (of the present), realizable.A: present (also but rarely- Present Perfect) after the if
B: Future Simple will/ can/ may/ might (or other modals)
If you learn, you will succeed.
If you have finished your dinner, I will wash the dishes.
Often refers to the present (either at the moment, or generally), unreal, less likely to happen- realizable at least in the future or in principle; to talk about hypothetical or imaginary cases.A: Past Simple (V2)
B: future past using the modals- would/ could/ might + V1
I have no idea were he’s living now. I’d call him if I had his phone number. (hypothetical)
If I were you (if I were in your place), I’d see a doctor. (advice)
If I won a lottery, I would buy a yacht. (hopes and ambitions)
Were is used as the form of the verb be for all persons. (However, in everyday conversation, “was” is often used except in the phrase If I were you,…)
If I were married, I’d move to a bigger house.
The modal could is used in the if clause as the past form of can.
If she could run five more kilometers, she would brake all records.
Refers to the past, unrealizableA: Past Perfect (had+ V3)
B: Future Past Perfect (would/ should/ could/ might+ have+ V3)
If you had come yesterday, you would have met her.
Both would and had can be shortened to ‘d:
If he’d been at the demonstration last week, I’d have seen him.