Yes same here, it's a little annoying to keep getting marked wrong for these things.

Objective/accusative case with a definite subject (indicated by the "the" definite article). "A fiú egy nagy fát fest" - "The boy is painting a large tree"; "A fiú a nagy fát festi" - "The boy is painting the large tree";

As you can see, the order of the words can be altered (my example vs the sample above), but that indicates the emphasis of the importance in the context, so: "A fiú festi a nagy fát" - stresses out the actionee, that it is the boy, who is painting the large tree (maybe there are others, who are also painting, but the men, women and a girl are painting a bush or something); and "A fiú a nagy fát festi" - Emphasising the subject, now it is a large tree the one which is being painted by the boy. (means, maybe he is in a longer painting process, during which he will cover various objects with paint, but now it's the tree's turn).