Which article do we have to use in this: "<...> then we watched TV, ate (the/a/-) Christmas cake and went to (the/a/-) garden". Why the other alternatives aren't (are?) acceptable?
It depends on whether you want to be specific or not. With the cake, you could use both, the indefinite article to show it was a single Christmas cake, but there are many others like it. the definite article would refer specifically to a cake we know about. The same would be true of the garden, though we tend to use the definite article as each house only has one.