Conditional Sentences

Conditional tenses 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 sentence construction using "unless" instead of "if".

There are various ways of classifying conditional sentences. One distinction is between those that state an implication between facts, and those that set up and refer to a hypothetical situation. There is also the distinction between conditionals that are considered factual or predictive, and those that are considered counterfactual or speculative (referring to a situation that did not or does not really exist).

Conditional sentence type	Usage	If clause verb tense	Main clause verb tense
Zero	General truths	Simple present	Simple present
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
Mixed type	An unreal past condition and its probable result in the present	Past perfect	Present contditional

Sumber : http://www.edufind.com/english-grammar/conditional/
               http://en.wikipedia.org/wiki/Conditional_sentence