Learn Russian: Из-за дождя он обычно остаётся дома один. - Because of the rain, he usually stays home alone.

The word один literally means "one", but it is commonly used to mean "alone". Even though "alone" is an adverb in English, in Russian it behaves like an adjective that describes the subject. Since the subject here is он (masculine), we use the masculine singular form один. If the subject were a woman, it would be она обычно остаётся дома одна.

Why use из-за here for "because of" instead of another word?

Из-за is a preposition specifically used to say "because of [a noun]". It always takes the Genitive case, which is why дождь (rain) becomes дождя. It is very commonly used for weather, negative reasons, or external obstacles. You cannot use потому что here, because that means "because" and must be followed by a full phrase with a subject and a verb.

Why is the word for "home" дома and not в доме or домой?

Дома is a fixed adverb that specifically means "at home" to show a location. If you said в доме, it would sound like you are emphasizing being physically "inside the building". You also cannot use домой here, because that means "to home" and is only used with verbs of motion (like walking or driving home). Since he is staying in a fixed location, дома is the correct choice.

Why is the verb остаётся (stays) in the imperfective aspect?