以下两种情况使用现在完成时：
1.表一个过去动作对现在的影响也就是影响性,它与过去一般时容易混淆.因为都表过去动作,区别是,一般过去时常有具体的过去性时间点作时间状语,强调在什么点上发生了---动作：
examples
1).I had my lunch at 12:30.
2).He came here half an hour ago.
现在完成时表的过去动作主要强调它多现在的影响,而不是在什么点上发生,因此不跟具体时间点,却常有already(早已）/yet(还没）just,before 等时间比较含糊的时间状语从句,
examples
1).She has already finidhed her competition./I haven't yet.
2).You have just told me about it.
3).We haven't heard of him before.
2.现在完成时还表持续性,这种情况下要用延续性动词表动作从过去的一个点一直持续到现在（可能还仍在持续着）常跟由since引导过去点状时间状语表动作起点,还可跟由for引导的线状时间状语表时间线段）
examples:
1.I have been a nurse since 10 years ago(I have been a nurse for 10 years)
2.We have lived in the town since we moved here two years ago/We have lived in the town for two years since we moved here.
3.He has been a mumber of the Party for 4 years.(这里be a mumber of 不能用非延续动词短语
