Conditional Sentence

Conditional tense are used to speculate about what could happen, what might have happened and what we wish would happen. In English, most sentences using the conditional contain the word if. Many conditional forms in English are used in sentences that include verbs in one of the past tenses. This usage is referred to as “the unreal past” because we use a past tense but we are not actually referring to something that happened in the past. There are five main ways of constructing conditional sentences in English. In all cases, these sentences are made up of an if clause and a main clause. In many negative conditional sentences, there is an equivalent sentences construction using “unless” instead of “if”.

Conditional Sentence Type	Usage	If Clause Verb Tense	Main Clause Verb Tense
Type 1	A possible condition and its probable result	Simple Present	Simple Future
Type 2	A hypothetical condition and its probable result	Simple Past	Present conditional or present continuous conditional
Type 3	An unreal past condition and its probable result in the past	Past Perfect	Perfect Conditional

Type 1 Conditional

The type 1 conditional is used to refer to the present or future where the situation is real. The type 1conditional refers to a possible condition and its probabale result. In these sentences the if clause is in the simple present, and the main clause is in the simple future

If Clause	Main Clause
If + Simple Present	Simple Future
If this thing happens	That thing will happen
If you don’t hurry	You will miss the rain
If it rains today	You will get wet

Type 2 Conditional

The type 2 conditional is used to refer to a time that is now or any time, and a situation that is unreal. These sentences are not based on fact. The type 2 conditional is used to refer to hypothetical condition and its probable result. In type 2 conditional sentences, the if clause uses the simple past, and the main clause uses the present conditional.

If clause	Main clause
If + Simple Past	Present conditional or present continuous conditional
If you went to bed earlier	You would not be so tired
If it rained	You would get wet
If i spoke italian	I would be working in italy

Type 3 Conditional

The type 3 conditional is used to refer to a time that is in the past, and a situation that is contary to reality. The facts they are based on are the opposite of what is expered. The type 3 conditional is used to refer to an unreal past condition and its probable past result. In type 3 conditional sentences, the if clause uses the past perfect, and the main clause uses the perfect conditional.

If clause	Main clause
If + Past Perfect	Perfect conditional or perfect continous conditional
If you had studied harder	You would have passed the exam
If it had rained	You would have gotten wet
If i had accepted that promotion	I would have been working in Milan

Source : http://www.edufind.com/english-grammar/conditional/