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