Unit 12 You are supposed to shake hands

五.語法知識
現在完成時的用法
have(has)+過去分詞
1.現在完成時用來表示現在之前已發生活或完成的動作或狀態，但其結果卻和現在有聯系，也就是說，動作或狀態發生在過去但它的影響現在還存在。
i have lost my wallet.（含義是：現在我沒有錢花了。）
jane has laid the table.（含義是：已可以吃飯了。）
michael has been ill.（含義是：現在仍然很虛弱）
he has returned from abroad.  （含義是：現在已在此地）
2. 現在完成時可以用來表示發生在過去某一時刻的、持續到現在的情況,常與for，since連用。
mary has been ill for three days.
i have lived here since 1998.
3. 現在完成時往往同表示不確定的過去時間狀語連用，如already, yet, just, before, recently, lately等：
he has already obtained a scholarship.
i haven’t seen much of him recently (lately).
we have seen that film before.
have they found the missing child yet?
4. 現在完成時常常與表示頻度的時間狀語連用，如often, sometimes, ever, never, twice, on several occasion等：
have you ever been to beijing? 
i have never heard bunny say anything against her.
i have used this pen only three times. it is still good.
george has met that gentleman on several occasions.
5. 現在完成時還往往可以同包括現在時間在內的時間狀語連用，如now, up to these few days/weeks/months/years, this morning/week/month/year, now, just, today, up to present, so far等：
peter has written six papers so far.
man has now learned to release energy from the nucleus of the atom.
there has bee too much rain in san francisco this year.
the friendly relations and cooperation between our two countries have been enhanced in the past few years.
up to the present everything has been successful.
6. 現在完成時表示現在之前業已完成的動作，雖然其效果或影響仍然存在但已不再繼續，但是有一些現在完成時的句子，在后面加上for+一段時間，則現在完成時的動作就表示延續性。
thomas has studied russian. (現在不再學俄語)
thomas has studied russian for three years. (=thomas began to study russian three years ago, and is still studying it now.（同第2點用法第一個例句）
7. 現在完成時還可以用來表示過去的一個時間到現在這段時間內重復發生的動作。
we have had four texts this semester.