If Conditional

Definition of a conditional clause

A conditional clause is a type of subordinate clause, most commonly introduced by the conjunction if or unless.  Like most subordinate clauses introduced by a conjunction, the conditional clause can either go before the main clause, or after it.

1. Open if clause - the open conditional statement :

This usually refers to a future event which is conditional on another future event.
The verb of the main clause is in the future tense with "will" (or sometimes another modal). The verb of the conditional clause is in the simple present tense. 

Example :

1. If you eat too much, you'll get fatter.

2. If I go to London, I can visit the British Museum.

3. If everyone works fast, we'll finish in time.

2. Type 2 if clauses - the open hypothetical conditional statement:

This refers to a possible future situation which depends on on another possible future situation. The verb of the main clause uses the present conditional tense (would + infinitive, or could+infinitive), The verb of the conditional clause normally uses the present subjunctive or preterite (these two tenses are identical except with to be). Occasionally, the conditional aspect of the statement can be emphasised by using the form were + to + infinitive. 

Example :

1.  If I went to London, I would / could visit the British Museum.

2. If you visited Scotland, you could see Edinburgh Castle.

3. Unless the directors increased sales, we'd have to close this shop.

3. Type 3 if clauses - the unfulfilled hypothesis 

This refers to a situation which an event might have taken place, but did not, because a condition was not fulfilled.  The verb of the main  clause goes in the past conditional (would have + past participle). The verb of the conditional clause goes in the past perfect (had + past participle).

Example :

1. If you had eaten too much, you'd (you would) have got fatter.

2. If I had gone to London, I could have visited the British Museum (but I didn't).

3. If you had visited Scotland, you could have visited Edinburgh Castle (but you didn't).

Source : http://linguapress.com/grammar/conditionals.htm