where I have to use aren't ? can I use "aren't" when I'm talking about clothes? example; " aren't this blouses a little expensive?" is it right?
Your use of aren't is correct, although you need "these blouses", not "this blouses". If you want to refer to a single item, you need to use isn't. Aren't is a contraction is are not, and isn't is a contraction of is not.
- Aren't these blouses a little expensive?
- Isn't this blouse a little expensive?
Mick probably gave you the answer you need, but a broader answer to when you can use "aren't" is that you can use it in present tense negative with the second-person singular and first-, second-, and third-persons plural:
Second-person singular: You aren't.
First-person plural: We aren't.
Second-person plural: You aren't.
Third-person plural: They aren't. (Also, "These blouses aren't.")
All of these become negative interrogative simply by reversing their order. And in a fluke of English, you can also use aren't in present tense negative interrogative with the first-person singular: "Aren't I?" Although, if you didn't use a contraction, you'd conjugate it as "Am I not?"