"Mary is trying to improve the condition of her house."

There seems to be confusions of forms and functions in the discussion of this example.

Any intelligent definition of the form of a verb phrase must recognize that a VP structure is finite. An infinitive is not a part of a true verb phrase except, as Bruce mentioned, as a modal equivalent ["have to" = "must," "be going to" = "will," etc.]. The confusion comes from catenative verbs, as Martha mentioned, where the semantics mixes stickily with the structures. Since an infinitive phrase can function nominally, adjectivally, or adverbially, the problems of putting a functional label on the infinitive form for a given example is not always just "a very basic exercise in part-of-speech recognition." 

Perhaps the confusion in the example is owing to the idiomaticness of "trying." In the example, a fair paraphrase of the verb "to try" seems to be "to attempt": 
    Mary is attempting to improve the condition of her house.
In that case, the infinitive phrase is seen more clearly in its function as a nominal/direct object, similar to using a noun phrase in its place:
    Mary is attempting an improvement to her house.
Using "to try" with an infinitive phrase following (whether you consider it catenative or not) is far more idiomatic, obviously, but the underlying structures are the same.

Paul Pellikka
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