1. If it isn't suitable, can I bring it back?
如果不合适的话，我能退货吗？
还能这样说：
Can I bring it back if it doesn't fit?
Could I return the goods if it isn't suitable?
应用解析：
suitable for/to sth. 适合的，适宜的，恰当的。
例如：Would it be suitable to have dinner now?
现在吃饭合适吗？
2. I'd like to return this.
我想退货。
还能这样说：
I want to get a refund.
I would like a refund of the product.
3. I'd like to exchange this blouse.
我要换这件衬衫。
还能这样说：
Could you exchange this blouse for me?
Would you like to exchange this blouse, please?
谚语：
A fair exchange is no robbery.
公平交易并非抢夺。
4. Why do you want to exchange it?
为什么要换货？
还能这样说：
Can I have the reason why you exchange it?
Why do you want a replacement?
应用解析：
exchange A for B 交换，互换，调换，更换；
例如：He exchanged the pen for a pencil.
他把钢笔换成了铅笔。