Conditionals
There are usually three types of conditionals that are popularly known as Type-I, Type-II & Type III or First,second & third conditionals.

Position of If-clause: There are two places where we can put the if-clause.Firstly,it can go at the beginning.If so,then we have to use commas after it.Secondly,we can use it at the end after main clause.

TYPE-1 If+ simple present, will
This pattern is used to mean future condition though if-clause is in the simple present. If you come to Thul, I’ll see you.

-After if-clause,an imperative sentence can be used.
If you reach late at night, don’t make a noise.
-Two simple tenses can be used in both clauses.
If Maria visits Karachi,she usually stays at me.

TYPE-2 If + simple past, would
This pattern is used for unreal and imaginary past events. This can mean “present or future”.
Note: Might & could can be used instead of would.
If I lost my job,I would do my own business.

- We say “If I were” in place of saying “If I was”. Some grammarians consider it (if I was) incorrect. However, it can be used.

TYPE-3 If +past perfect, would have
We use this pattern for some unreal past events that didn’t happen. We can also construct main clause by using could have & might have.
-If you had worked hard, you would not have failed in the B.A. exam.

-Besides these three conditionals, we also use if (or when, etc) with other words for the different meaning. We sometimes become confused to find such sentences. Some important patterns and points are being given below:

(1) If + should & If + happen to: We use this pattern for something that is unlikely or unexpected.
- If you should/ happen to go to the market, buy some fruit.

(2) If + was/were to: This structure has the same meaning as the second conditional has.

(3) If so & If not: To avoid repetition, we use these forms.

(4) If = then: We normally consider the use of then after if clause incorrect. However, it isn’t so. We can construct sentences with it to put emphasis.

(5) Would in both clauses: Sometimes we get puzzled to face the sentences in which would has been used in both clauses. Normally, they are accurate.
-What would you do if you would lose your purse?

Reference: Practical English Usage by Michael Swan. Entry No.256 to 264 (page No.233)