[clivehawkins]

Hi Mark;
Long time! Finally back after months of sleepless nights (and sadly i'm not exaggerating). In fact, let's use sleepless nights and newborn babies as an example :-)

When using the past of can ie could, we can only use it in positive sentences when it refers to a general situation, rather than a specific one-off.
Here are two specific situations:
Last night I was able to make my son sleep :-) - correct.
Last night I could make my son sleep :-) - incorrect, because it's a specific situation.

Here are two general situations:
When I lived in London I was able to get around very easily - correct.
When I lived in London I could get around very easily - correct.

For negative sentences, you can use be able to or could for both specific and general situations:
Last night I wasn't able to sleep because my son has a cold - correct.
Last night I couldn't sleep because my son has a cold - correct.

When I was a child I wasn't able to swim - correct.
When I was a child I couldn't swim - correct.

I hope that helps.
Speak soon
Clive