Conditional Sentences in English

"if" is used when the time we refer to is now or always and the situation is real and possible. This type of conditional is usually used to talk about general facts. The verb tense in both clauses is the "simple present". In type 0 conditional sentences, the term "if" can normally be replaced by "when" without changing the meaning.

If this thing happens                          that thing happens.Type 1 conditional is used to refer to the present or future when the situation is real. Type 1 conditional refers to a possible condition and its probable result. In these sentences, the "if" clause takes the "simple present" and the main clause takes the "simple future".

If + simple present                         simple future                     

Type 3 conditional is used to refer to a time in the past and to a situation contrary to reality. It is based on facts that are opposite to what is being expressed. Type 3 conditional is used to refer to a past condition that is not real, as well as its probable result in the past. In these sentences, the "if" clause takes the "past perfect" and the main clause takes the "perfect conditional".