A dog in the forest.
The dog is in the forest.
The dog is in the forest is a more specific and grammatically correct way of saying a dog in the forest.
exists x1 x2.(n_forest(x2) & a_topic(x1) & in(x1,x2) & n_dog(x1))
exists p1 x1 x2.(n_forest(x2) & n_dog(x1) & in(x1,x2))