现在完成时之瞬间动词转变成延续性动词
for\since:
for+一段时间，常常可以发现for+数字
，for three months for a long time for many years
Since+时间点，常常可以发现
a. since+月份，年份，表示时间的几点
since September since 1990 since 7 o’clock
b. since+一段时间+ago；
since two years ago since ten months ago
+ last+....
since last month since last year
现在完成时的结构式has\have done 请注意第三人称：she\he\人名...+has
当句子为肯定句，且居中出现for\since的时候，句中的时态要用现在完成时，且句中的动词要用延续性动词。
当句子为否定句时，句中的动词可以为瞬间动词，表示某个动作不发生的状态已经持续了......
在有for\since的现在完成时的从句中，如何将瞬间动词转换成延续性动词
以borrow——keep为例子
They borrowed it last week.
They have kept it since last week.
分析：
我们已经知道现在完成时的标志就是has\have done，因此第一步就是看主语，主语时They，我们选择have done。
因为borrow——keep.而keep——过去分词：kept，因此，have borrowed就需要改成have kept. Since后面只能接时间点，而last week就是一个时间点，因此，since后面直接填写last week。常考的瞬间动词：
原形过去式
过去分词动词原形动词过去分词die--be dead die died
died be dead been dead borrow--keep borrow borrowed borrowed keep kept leave--be away from
buy--have buy bought
bought have had
begin--be(on) begin began
begun be been
join--be a member of
be in join joined joined be in been in become--be become became become be been
Open\close--be open open opened opened be open been open
finish--be over finish finished
finished be over been over
put on--wear put put
put wear worn
catch a cold--have a cold catch caught caught have had
go to--be in go went
gone be in been in
get to know--know get got got know known
Arrive in\at--be in arrive arrived
arrived be in been in
Move\come\get to--be in
go out--be out go went gone
be been
Move\come\get to--be in
be in been in
举例
Mr. Smith came to this school in September.
Mr. Smith ___has____ ____been____ at this achool_____since___ September.
分析：
1, come--came--come
2, 本来应该是填写has come to，但是come to要转换成be in
3, 而be的过去分词为been因此我们填写的内容为has been in
4，因为句子中已经有了at，所以我们只需要填写has been
