Didn't implies you could have done it but chose not to. Couldn't is when you wanted to do it but weren't able to. You didn't do the lawn because you didn't feel like it. You couldn't do the lawn because it rained. Does that make sense? Hope it helps. :)

Sadly I can't nuke my dumb response. Linleemill's below makes a lot more sense than mine did so I removed it.

'could' is the past tense of 'can' - an ability to do something. (verb) 'I can count to a million' - present (positive) 'I could only count to 100 when I was a small child' - past (positive) 'I can't (cannot) catch the bus' - present (negative) ' I couldn't (could not) catch the bus' - past (negative) 'did' is the past tense of 'do' - an activity - when you're doing something. (verb) 'I do judo once a week' - present (positive) 'I did judo last year' - past (positive) 'Do you catch the bus every day' - present (positive) ' I didn't (did not) catch the bus this morning' - past (negative). I hope that helps :)

didn't - possible, could have couldn't - impossible If you missed the bus because you decided to stay home, or took the train, or arrived at the station too late, that's didn't. If you missed the bus because you broke your leg, or there was a snowstorm, or there was no bus, to catch that's couldn't.