动词的一般过去时形式 Regular or irregular past simple forms
本文从英语过去时的距离性实质出发研究一般过去时 （包括情态动词的过去时形式 ）用来表示客气礼貌的语用机制。
The present dissertation investigates pragmatic constraints on preposing constructions in the acquisition of English as a foreign language (EFL) by Chinese-speaking learners.
但事实上，要是我来选的话，我会选择a为正确答案，因为它是一般过去时的形式，与句中的bought、yesterday和didnt的时态保持一致。
Actually, I would have selected a as the correct answer because it is past tense in keeping with the sentence - "bought", "yesterday" and "didn't".
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