Uncertain present condition

(與現在事實不確定)

simple present tense in both clauses

Uncertain future condition

(與未來事實不確定)

simple present tense in the if clause

will or be going to + base from verb in the result clause

Contrast to present condition

(與現在事實相反)

simple past tense in the if clause

would (could, should or might) + base form verb in the result clause

Contrast to past condition

(與過去事實相反)

past perfect in the if clause

would (could, should or might) have + past participle in the result clause