Final answer:
In the sentence provided, the word 'tornadoes' is a noun, which is the subject of the sentence and refers to a specific weather phenomenon.
Step-by-step explanation:
The word 'tornadoes' in the sentence 'Tornadoes usually form from a dark cloud in the sky.' acts as a noun. It is the subject of the sentence and refers to a type of weather phenomenon characterized by a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud.