ANOTHER REVIEW OF MODAL VERBS-6

Hello again everyone. Welcome back to the Daily Vitamin. 

What is the difference between the following sentences?

1) I can speak German.
2) I am able to speak German.

There really is no difference in meaning, but the modal verb can for ability is more common in spoken English.

However, can doesn't have different forms for other tenses. Therefore, we often use the "periphrastic" modal verb be able to to express ability in other verb tenses.

3) I haven't been able to practise my German this week because I have been ill. (present perfect)
4) I will be able to practise my German tomorrow. (future)
Etc.

Remember that the past of can is sometimes represented by could, and we also have the option of using be able to.

5) I couldn't understand you when you were speaking to me.

6) I wasn't able to understand you when you were speaking to me.

Could as the past of can for ability is especially common with verbs like see, hear, feel, etc. or to talk about general ability in the past.

7) Yesterday I could see how much you were studying your German.

NOTE: Sometimes it is not possible to use could as the modal verb for ability in the past and we must use be able to. It is necessary to use be able to when the ability being expressed means to overcome a difficulty (supererar una dificultad).

8) Although it was difficult, yesterday I was able to finish the article I was writing.

In example 8, I ovecame a difficulty and finished writing my article. In this case, could would not work (**Yesterday I could finish the article...**) since could in the past only works with general ability, not for overcoming a difficulty.

The negative couldn't is possible in all situations: general ability and overcoming difficulties.

9) Yesterday I couldn't finish the article I was writing. (overcome)

10) When I was 4 years old, I couldn't write. (general ability)

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Have a great day.