牛津版八年级下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...