Learn Italian: Il dormire molto è tipico dell'orso. - Sleeping a lot is typical of the bear.

This is called a substantivized infinitive, which means we are using a verb as a noun. In English, you usually use the '-ing' form for this (like 'sleeping'). In Italian, you use the masculine singular definite article il followed by the infinitive of the verb. It turns the action into a noun concept: the act of sleeping.

If il dormire is acting as a noun, why does it use the adverb molto instead of an adjective?

That is the unique trick of substantivized infinitives! Even though they act like nouns in the sentence (taking an article and acting as the subject), they still keep their verb-like properties. Because dormire is still fundamentally a verb, it is modified by an adverb (molto) rather than an adjective like tanto.

Could I just say Dormire molto è tipico... without the il?

Yes, you can. Dropping the article is perfectly grammatically correct and very common in everyday speech. Adding il makes the sentence sound just a bit more formal, or it emphasizes sleeping as a specific, defined concept.

Why does the sentence use the singular dell'orso instead of the plural degli orsi (bears)?