If you ________(not be) careless,your mother will be very happy.

问题描述:

If you ________(not be) careless,your mother will be very happy.
答案是aren't ,请问为什么不能用don't

are是系动词,后面跟形容词的careless作表语;而do是实意动词“做某事”的意思,后面要跟名词或代词作为它的宾语,形容词是不能作宾语的.