牛津8B Unit4期中复习课件（共17张ppt）
现在完成式的标志：
1. 不确定的时间：yet, just, recently, lately
2. 频率：once, twice, ...times
3. 包含现在的一段时间：till now, up to now, so far in/during/over/for the past/last years,
4.for+段时间 since+点时间(last May)/过去式（I was born）
it is/it has been five weeks since...
5, It is the first/second... time that...
6. It is the best/worst...that...