in which sentences we should ask isn't aren't

isn't is used with singular noun and pronoun and aren't is used with plural noun and pronoun.it is used in assertive,interrogative sentence and for question tag.

Is is singular and Are is plural. With plural subjects use 'are', with singular subjects use 'is'. (he, she, it , that_ singulars) (I, we, you, they, those, and these are plurals)