-/The oxis a strong beast of burden.
The oxis- we are thinking of the animal as a type so we use the article and there are many beasts of burden, of which it is one, hence the indefinite article.
-/Snow covered all the ground
The ground here is specific- we know where you are talking about.
-/the apple trees in the garden stood in full bloom
We knpow which apple trees you are talking about