Can - what part of the word?
Words such as "can" in the Russian language express a state that means binding, possibility, necessity. Call these words as predicatives - words with modal meanings of duty, necessity, opportunity.
Understand the "modal" value should not scare you, because it means the individual attitude of the speaker to the information presented to them.
At school, these words are not studied as a special part of speech, so to the question: can it be - what part of speech? It is worth answering that it is a predicative or a word of a category of state.