tree
The German word for tree is 'Baum'. It is a masculine noun and is used in the same context as the English word 'tree'. For example, 'Der Baum ist groß' translates to 'The tree is big'.
The tree is very big.
This sentence is a simple declarative statement in the present tense, stating a fact about the size of a tree.
Under the tree is a dog.
This sentence is using a location preposition 'unter' (under) to indicate the position of the dog relative to the tree.
The bird flies to the tree.
This sentence is expressing an action in the present tense, where the subject (the bird) is performing an action (flying) towards a destination (the tree).