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现在完成时是表示过去的动作与现在的联系，主要说明的是现在的情况和状态；而一般过去时则强调动作发生在过去某一时间，与现在不发生联系
We haven't seen him since last year. (我们自从去年以来一直未见到他．（现在还未见到）
We didn't see him last year. 我们去年没见到他（现在不一定未见到）
现在完成时表示从过去发生一直持续到现在（包括现在）的某个动作或状态，与现在有关的时间状语连用，一般过去时表示过去某一特定（与现在无关）的时间，与具体的表示过去的时间状语连用
Mr. James Scott has a garage in Silbury and now he has just _ (buy) another garage in Pinhurst. Pinhurst is only five miles from Silbury, but Mr. Scott cannot get a telephone for his new garage, so he has just _ (buy) twelve pigeons. Yesterday, a pigeon _ (carry) the first message from Pinburst to Silbury. The bird _ (cover) the distance the first three minutes. Up to now, Mr. Scott has _ (send) a great many requests for spare parts and other urgent message from one garage to the other. In this way, he has _ (begin) his own private “telephone” service.
