did是助动词do和does的过去式,而was/were是be动词am、is/are的过去式。
一个句子中,如果没有be动词,发生动作要用助动词do或does.
例如:I am a student. 问句是:Are you a student? 有be动词所以直接用are提问。
I worked in an international company 2 years ago.
句中没有be动词,所以要用助动词代替,问句是:
Did you work in an international company 2 years ago?
明白了么?
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