No, it is not. It is an adverb or a preposition (used with an object, e.g, underneath the bridge).
Underneath
Yes, it is, meaning underneath the surface. Underground can also be a noun.
underneath = unterhalb
He walked underneath the branches.
Yes, it is. Underneath, like beneath, can also be a preposition (underneath something) but can be used without an object. (e.g. He lifted the rock and looked underneath.)
No, it is a verb or a noun (to go around, to surround; a round shape). The adjective form is circular.
It is an adjective.It is a an adjective.
Cautious IS an adjective. An adjective is an action!
An adjective
it is an adjective!
No. It is not an adjective. An adjective describes something.
No, it is an adverb. The adjective is clumsy.