I have three questions to ask someone.Come in for help me.

13)其實a long time和for a long time均正確，在牛津第六版就曾討論過：
In negative sentences (for) a long time sometimes has a different meaning from (for) long...而它給與的例子就是：
I haven't been here for a long time（留意：在這條的討論中括號中的for表示可以省略）
而牛津出版的”Practical English Usage”在討論long與for a long time使用的分別時，也有以下的例子：
She didn't speak for a long time.
可見for a long time是絕對正確，而你的習作只給a long time，而否定for a long time那也不正確；注意：練習有時並不可信，你大可問你的老師，並提出牛津第六版字典，也有用for a long time的例句，相信你老師也會看出練習的問題。
16.
I have been to London意思是：曾經到過倫敦
而I have gone to London.意思是：已經去了倫敦
兩句意思是完全不同。

１６．was...could
因為sure是形容詞，而配was等動詞，而不是助動詞did；若用did句子就沒有動詞。
希望幫到你！

13. - Did you get any mail? - No, I haven't gotten a letter ____a long time____ .
(A) a long time before
(B) since a long time
(C) for a long time
(D) a long time
why the answer isn't for a long time??Because you don't need that extra word to informA long time already meant that sentence.

16. I __have been______ to London several times.
(A) go
(B) have been
(C) have gone
(D) went
why the answer isn't for have gone??Because To already substitute the word GONE.

16. - What about Peter? - He said he ____was____ not sure he ___could_____ come.
(A) did - could
(B) does - can
(C) was - could
(D) was - can
why the answer isn't did could??you can't say"He did not sure"it's "he was not sure" and was and could ia past tense and "can" is present tense.Therefore,you have to stay in past tense,which in this case,you should use"could"





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