No part of "tooth decay" is a structure or function.
part of speech
The part of speech for this particular word is a noun.
Each isotope has a specific radioactive decay.
adverb
what part of speech is work
what part of speech is beneath
the part of speech sashay is a averb
Did is a verb, and not is an adverb. Didn't is not any part of speech. It's a contraction of did and not.
Adjective
The word speech is a noun.
No, but it's part of a noun which the noun is the part of speech.