Learn Spanish (Latin America): No puedo dormir porque hace mucho calor.

In English, we say "it is hot", but Spanish uses the verb hacer (to make/do) for many weather conditions, including heat, cold, and wind. You are literally saying "it makes heat". Using está or es here would be incorrect.

The translation says "very hot", so why does the sentence use mucho instead of muy?

This is a very common trap! In Spanish, calor is a noun meaning "heat", not an adjective meaning "hot". Because you are literally saying "it makes a lot of heat", you must use mucho (much / a lot of). You only use muy (very) to describe adjectives.

Why is dormir in its infinitive (dictionary) form instead of being conjugated?

When two verbs are used together in a phrase like this, you only conjugate the first one. Since puedo (I can) is already conjugated to match "I", the second verb dormir (to sleep) stays in its basic infinitive form.

Is porque always written as a single word?

Yes, when it means "because", it is always written as one word with no accent: porque. If you are asking a question and want to say "why?", it changes to two words with an accent mark: ¿por qué?.