A conditional sentence tells what would or might happen under certain conditions. It most often contains an adverb clause beginning with ‘if’ (expressing the condition in which the other clause would be true) and an independent clause usually including a modal verb like ‘will,’ ‘would,’ ‘could,’ or ‘might.’
Either clause can go first: if it’s cold, I’ll wear a jacket” or “I’ll (I will) wear a jacket if it’s cold.” “If Johnny had studied, he would have gotten a better grade” or “Johnny would have gotten a better grade if he’d studied.” (See Complex Sentences for more on independent and dependent clauses.)
There are four kinds of conditionals:
The zero conditional is present tense and expresses a fact. It tells about something that always happens under certain conditions.
The 1st conditional tells what may happen in the future if a certain condition is met:
The 2nd conditional imagines what would happen if something were true-- but it’s not. This is often called the “present unreal conditional.” The verb after the ‘if’ is in simple past form, even though it’s talking about the present. This tells listeners that the suggestion is imaginary, not true.
(In formal use the verb ‘be’ is also in plural form, even for singulars, as another clue. See “if something were true” above. Speaking casually, many people use ‘was.’)
The 3rd conditional talks about how the past might have been different if different steps had been taken. The verb after the ‘if’ is again farther into the past-- in this case, a past perfect, compared to a conditional perfect using would or another modal in the independent clause.
For a little more about how to use 'will,' would,' 'might,' 'could,' and other modal verbs that can help form conditional tenses, see Modal Verbs.