1. can, could, be able to Look at these four sentences. Why don’t we use a form of can in the last two? Twenty years after Mr Moore’s first successful climb of Mount Everest he has decided to repeat the endeavor. He cannot climb to the top of Mount Everest without an oxygen tank. His wife couldn’t imagine that he would want to make such a climb. He will never be able to reach the summit without a guide. He used to be able to make such climbs without oxygen.

2. canbe able to Present simple Future (will) Past simple* Present perfect Infinitive Gerund can No could* No am / is / are able to will be able to was / were able to has / have been able to (to) be able to being able to

3. Use can / could + infinitive To say that something is / was possible To say that something or someone has / had the ability to do something

4. Use be able to When there is no form of can (e.g. present perfect, future, infinitive)

5. Compare the use of could and was able to Use could or was able to to talk about a general ability in the past: My sister could / was able to play the violin when she was only 5 years old. Use was able to to talk about an ability in a specific situation in the past, although it was difficult: My sister was able to complete the concert with a broken bow!