Could vs. Was Able To

These two phrases are almost the same in meaning, but there are some cases in which one sounds more natural than the other.

In particular, when you are emphasizing something that you actually did, was able to sounds better.

Strange:

• Even though the train was crowded, I could get a seat.

Better:

• Even though the train was crowded, I was able to get a seat.

Could is not incorrect in the example above, but was able to sounds more natural and emphasizes the fact that you actually did get a seat.

This is probably because could is more often used to describe events that did not actually happen, but that might have happened if the situation had been different.

In other words, could is often used to describe counter-factual events. The next sentence is an example of this.

• I could get a seat but I'd rather stand.

Here, the writer's is emphasizing that there are empty seats available if he wants to sit; however, he is choosing to stand instead .

• I knew that if I got to the station early I could get a seat, but I didn't want to get up so early.

In this example, the implication is that the writer actually did not get up early, did not get to the station early, and so did not get a seat.