Uncertain present condition

(與現在事實不確定)

am, is, are in the if clause

simple present tense in the result clause

Uncertain future condition

(與未來事實不確定)

am, is, are in the if clause

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

Contrast to present condition

(與現在事實相反)

were 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)

had been in the if clause

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