could與 was/were able to 的用法...

Question

書上有幾句句子,可是我分不清什麼時候可以用could什麼時候要用was/were able to,書上也沒詳細的說明所以也不知書上寫的對不對,請文法達人幫我解答吧!不要網路上的答案因已查過還是不了解謝謝

1. I was able to get home before seven last night.

=>書上註明用I could get home是錯誤的,為什麼不能用could呢?


2. I was able to reach him on his cell phone yesterday.

=>這句was able to可以用could代替嗎?


3. I couldn't get home before seven last night.

=>這句為什麼又可以用couldn't呢?那也可以用wasn't able to代替嗎?


4.I couldn't reach him on his cell phone yesterday.

=>這句為什麼又可以用couldn't呢?那也可以用wasn't able to代替嗎?

補充:

1.我昨天晚上可以在7點前回到家
2.昨天我可以用手機聯絡上他
3.我昨晚無法在7點回到家
4.昨天我無法用手機聯絡上他


希望有多點人回答我會選出最棒的答案^^

Answer 1

我只是「看書回答」囉！工具書很多，可以多參考。試回答如下：

be able to比can還「正式」，有些時候通用，有些時候不行。回答你的問題如下：

1. I was able to get home before seven last night. =>書上註明用I could get home是錯誤的,為什麼不能用could呢?
答：was able to 是指「過去的能力」而且「已經做了」，故上句表「昨晚我有辦法在七點前回家(已回家)」，could無法表達「是否已做到」。could get home只是「有能力到家」但沒有「真正回到家」之意。

2. I was able to reach him on his cell phone yesterday.=>這句was able to可以用could代替嗎?
答：不行。理由同上。請記住was able to的含義：had the ability to do something and also did it（那時有能力做，且做了）而could只有had the ability to do something之義。

I couldn't get home before seven last night.=>這句為什麼又可以用couldn't呢?那也可以用wasn't able to代替嗎?
答：couldn't及wasn't able to皆表「過去沒有能力做…」故可通。

I couldn't reach him on his cell phone yesterday.=>這句為什麼又可以用couldn't呢?那也可以用wasn't able to代替嗎?
答：同上句。

could也可表示「現在可能做，但實際上沒做」例：We could go home.我們可以回家的(但實際上沒回）

could have+pp表「過去有能力做，但實際上沒做」例：We could have gone home.我們那時可以回家的（但那時候實際沒回）

以上參考Collins Cobuild出版English Usage。

蝦米小居，初學英語學習網站。
www.sammiwago.idv.tw

Answer 2

我不是文法達人, 所以有錯請指正.

一般用法裡, can/could 可以和 be able to 交替使用, 這是沒錯的, 不過若是語句帶有與過去事實相反的意思, 那情況就不一樣了. 與過去事實相反的話, 應該是要用 could + have pp.

1. I was able to get home before seven last night.
=>書上註明用I could get home是錯誤的,為什麼不能用could呢?
翻譯成, 我本來可以在昨晚七點前回到家的, 暗喻昨晚七點後才回到家.
應該用 I could have gotten home blah blah blah.

2. I was able to reach him on his cell phone yesterday.
=>這句was able to可以用could代替嗎?
建議解答和理由同 1.

3. I couldn't get home before seven last night.
=>這句為什麼又可以用couldn't呢?那也可以用wasn't able to代替嗎?
這句是直述句, 直接說明我昨晚七點前沒辦法回到家, 所以可以用 couldn't. 這邊就可以用 was not able to.

4.I couldn't reach him on his cell phone yesterday.
=>這句為什麼又可以用couldn't呢?那也可以用wasn't able to代替嗎?
建議解答和理由同 3.