"І drank a lіttlе coffее and was ablе to work tіll mіdnіght" - the whole thing takes place in the past, and you did work until midnight І drank a lіttlе coffее and could be working tіll mіdnіght. - The work is taking place in the present. It is possible that you will work until midnight, but you are not sure if that will happen.

In the 1st statement, you actually drank the coffee and you actually worked until midnight. Both actions are complete. In the 2nd statement, you drank the coffee but it is early and you speculate that you might be able to work until midnight. You are still working.

In the first sentence, the person was saying that because of the coffee they were able to stay awake. In the second sentence, the person was saying that the coffee may be the reason why they are still awake.