When we want to express a condition in English, we use the conjunction IF:
- If you need a bicycle, I can give you mine

also reverse:

- I can give you my bicycle if you need one.

The if-clause = If you need a bicycle
The main clause = I can give you mine

A conditional sentence expresses this idea: when the if-clause is true, the main clause is also true, but when the if-clause is not true, the main clause can't be true.

- if you want some water, I will give you a glass:
possibility 1- you want some water. I give you a glass
possibility 2- you don't want some water. I don't give you a glass

The verb in the if-clause is usually different from the verb in the main clause. The combination of tenses is almost "infinite", but there are 4 combinations that are more usual than the rest, and we are going to study those combinations here. We call them "types".

TYPE 0 (eternal conditions) present + present
We use the simple present in the if-clause and also the simple present in the main clause.
We use this type to talk about something that is always true (for example, a scientific fact).

- If you drop a stone, it falls down to the ground    (this happens now, in the past and in the future)
- If you heat ice, it melts

TYPE 1 (future conditions) present + will / 'll
We use this type to talk about a condition that may happen in the future (we are talking about a future situation)

- If you come, I will tell you that  = If you come here in the future, I will tell you that in the future
- if you need a car tomorrow, I will give you mine

TYPE 2 (present condition) past + would / 'd
We use this type to talk about a present situation that is impossible (or it seems impossible to us)

- If he needed something, he would tell me = but he doesn't need anything, so he doesn't tell me
- If I had a lot of money, I would buy a bigger house = but I don't have a lot of money, so I can't buy a bigger house
- If I had the time, I'd visit you today = but I don't have the time, so I can't visit you today

TYPE 3 (past condition) past perfect (had + past participle) + would have
(if your level of English is low, you don't need to study this type)
We use this type to talk about a past situation that never happened

- If I had known you were coming to visit me, I wouldn't have gone away = but I didn't know, so I went away
- If she had been rich, nobody would have sent her out of the party = but she wasn't rich, and they sent her away


MIXED TYPES

Apart from those 4 types, in real English we can find just about any combination, depending on the situation. For example:

If-Present + imperative= If you see my sister, tell her to come home.
If-Present + going to= If you pass your exams, we’re going to have a party.
Would + If-passive past= What would you do if your house was destroyed.
If-Present + would= If I call the police, would you run away?
If-Present + may= If you leave the door open, someone may come in.
Should + If-Present= What should I do if I hear the fire alarm?
If-Present perfect + present= If he has seen this movie, I’m sure he likes it.