Happy is the main lexical predicate here. It takes one argument: an experiencer--John. Tall takes one argument: a theme: Mary. (12) *John is being tall. (13) *Mary is being fond of red roses. (14) ...

If the verb in the predicate of a sentence expresses the existence of something, such as IMASU (to be, to exist), NI indicates where something is or exists. For example, "I am at the station" is ...