Answer:
Infinitive Phrase: to spend the afternoon tobogganing.
Use: As an adverb.
Step-by-step explanation:
An infinitive phrase is a verb phrase that contains the word "to" + the base form of the verb. This means that an infinitive phrase is a phrase with the construction "to+ V1 + modifiers".
In the given sentence "Tom ad Tim decided to spend the afternoon tobogganing", the infinitive phrase is "to spend the afternoon tobogganing". Here, the "to+V1" form is the infinitive that describes what the subjects "Tom and Tim" are doing or decided to do.
And the infinitive phrase in this sentence acts as an adverb as it describes the verb. The easiest way to identify an infinitive phrase as an adverb is if the sentence answers the question "where, when, why, how, for what reason" etc.
Thus, the correct answers are
Infinitive Phrase: to spend the afternoon tobogganing.
Use: As an adverb.