Meet some problem

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Although I'm in a special period, taking the final examination,I still surf on internet from time to time.It is because I don't think the final exams are so difficult and needn't to spend so much time on it. Last night,I join a QQ group where is used to practice our oral english.But I was laughted at by a member for a sentence which I said that is"I don't know to say what at here."I don't know what's the problem with this sentence.As a sentence of oral english,I don't think there is any problem with it. My friends on mylot,could you help me solve the problem?could you point out the mistake in this sentence?Thank you in advance!