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2.3.2. "Could" and "be able to"
"Can" of the modal auxiliary has the meanings of the ability, the possibility and the permission. "Be able to" also has the ability but it has an actual act unlike "can", this difference is seen with the past tense. See the following examples.

a. I can swim.
b. I can speak English.
c. I'm able to swim.
d. I'm able to speak English.

The above is the present tense. (25a) and (25b) use "can", and these are changed into each (25c) and (25d) with "be able to." With the present tense, both of them has no difference because it can not express the completive acts and they express only the ability. (*10)

*10 The present tense can express the completive acts as the instantaneous act. (Cf. 1.1.2.) But with "can" it can not express instantaneous. Therefore, to express the present, "can" is preferred to "be able to."

Time feeling

But with the past tense, the difference is occurred because of the completive act. See the following examples.

a. I could swim.
b. I could speak English.
c. I was able to swim.
d. I was able to speak English.

Even if the past tense is used, if we treat them as the ability, these are the same, but if we treat them as the actual act, (26a) and (26b) can not be used. See another example.

After she studied hard, she was finally able to pass the entrance exam.

The above uses the past tens of "be able to" and means she took the exam and passed it, she was happy. In the case of this, "could" can not be used. If it is used, the result of the exam is unknown because only the ability passing exam is occurred.

Moreover, in a negative sentence, both of them can be used because the acts are not occurred. For example, "I couldn't swim." means the two either it was impossible to swim or it was not done actually.

Time feeling

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