English Grammar : Can, Could, Was Able, etc.

Can has two main uses :

1. To express permission or possibility (=may).
Example : You can go now.

2. To express ability or capacity (= know how to).
Example : I can swim very well.

Form	1	2
Future	Can	shall (will) be able to
Past	Could	could or was (were) able to

Say the following using the given time-expressions, first for the future and the for the past.

Example :
You can drive. when you are 17/after you got your license.

Answer 1 : You can drive when you are 17.
Answer 2 : You could drive after you got your license.

I can fly a plane. after a few more lessons/when I was in the air force.

Answer 1 : I can fly a plane after a few more lessons.
Answer 2 : I could fly a plane when I was in the air force.

Seeing in a grammar book that could can be past or conditional of can, students are always puzzled by having such sentences as :

• "I could pass my examination ten years ago."
• "I could go to the country yesterday and had a good time,.." etc.

corrected to was able to.

A careful analysis of all sentences where we cannot use could as simple past tense of can shows the following idea to be common to all of them. They deal with the attainment of something through some capacity. Mere capacity may have could or was able to.

• He could (was able to) swim very well when he was young. (Could is more usual)

But something attained through a capacity may not have could.

• He was able to swim half-way before he collapsed. (Could is impossible here)

Managed to also expresses this idea.
The past of can meaning permission always has could.

• I could put it wherever I liked. (permission granted)
• I was able to put it on the top shelf. (capacity or ability to reach)