Zero Conditional, 1st Conditional, 2nd Conditional and 3rd Conditional

Zero conditional

Structure: If/When/Unless+ Present Tense        ////       present tense / imperative

• If the sum of the digits of a number is divisible by three, the number is divisible by three (Pres. tense)

• If you see Mr Fox tonight, tell him I am ill. (imperative).

When to use it:  
Nature: When we talk about things that are generally or always true
Time: Present time and imperative

1st conditional

Structure: If + Present Tense        ////       will + infinitive

• If you help me with the dishes (if + pres), I will help you with your homework. (will + inf)

When to use it:  
Nature: Open condition, what is said in the condition is possible.
Time: This condition refers either to present or to future time. e.g. If he is late, we will have to go without him. If my mother knows about this, we are in serious trouble.
More Info  1st conditional

2nd conditional

Structure: If + Past Tense       ////       would + infinitive

When to use it:  
Nature: unreal (impossible) or improbable situations.
Time: present; the TENSE is past, but we are talking about the present, now. e.g. If I knew her name, I would tell you. If I were you, I would tell my father. Compare: If I become president, I will change the social security system. (Said by a presidential candidate) If I became president, I would change the social security system. (Said by a schoolboy: improbable) If we win this match, we are qualified for the semifinals. If I won a million pounds, I would stop teaching. (improbable)
More Info 2nd Conditional

3rd conditional

Structure: If + Past Perfect Tense       ////       would have + Past participle

We do not normally use will or would in the conditional clause,
only in the main clause.

When to use it:  
Nature: unreal
Time: Past (so we are talking about a situation that was not so in the past.) e.g. If you had warned me, I would not have told your father about that party.(But you didn't, and I have).
More Info 3rd Conditional

EXERCISES:

• Ex. 1: Zero and 1st Conditional.
• Ex. 2: Zero and 1st Conditional
• Ex. 3: Zero and 1st Conditional
• Ex. 4: Zero and 1st Conditional

• Ex. 5: 2nd Conditional
• Ex. 6: 2nd Conditional 
• Ex. 7: 2nd Conditional  
• Ex. 8: 2nd Conditional 

• Ex. 9: 3rd Conditional  
• Ex. 10: 3rd Conditional 
• Ex. 11: 3rd Conditional
• Ex. 12: 3rd Conditional

• Ex. 13: Mixed Conditional
• Ex. 14: Mixed Conditional

Mainly modified from http://grammar.ccc.commnet.edu