The infinitive phrase is "to spend the afternoon tobogganing".
An infinitive phrase expresses the action in a sentence. They can often function as nouns, adjectives or adverbs. E.g.: To win the approval of her mother-in-law by cooking dinner was Mary's only hope. The infinitive phrase in bold functions as a noun because is the subject of the sentence. When the phrase explains 'how' it functions as and adjective, when the phrase explains 'why' it functions as an adverb.
His academic decline is due to his excessive independence.
Explanation:
James used to have bad behaviour as regards behaviour. He was in prison for at least 6 years, and this lead to develop a kind of independent behaviour that was put into academic issues.
