时态讲解：现在完成时（1）构成：现在完成时由助动词have + 过去分词构成，助动词have 有人称和数的变化。第三人称单数用has，其余用have.现在完成时的否定式直接在助动词后面加上not、疑问式是把助动词提到主语之前。以study 为例，其否定式、疑问式和简单回答形式如下：否定式疑问式I have not (havent) studied.Have I studied?You have not (havent) studied.Have you studied?He has not (hasnt) studied.Has he studied?否定疑问式简单回答（肯定/否定）Have I not (Havent I) studied?Yes, you have. No, you havent.Have you not (Havent you) studied?Yes, I have. No, I havent.Has he not (Hasnt he) studied
