Depending on the context, struggle can be a noun or a verb.
As a noun: Raising a child can be an emotional struggle.
As a verb: He struggles to get the oversized TV into the too-small trunk.
In this sentence, "weakly" is an adverb modifying the verb "struggled." It describes how the flounder struggled.
"Lost" is being used as an adjective to describe the knight in this sentence.
part of speech
The part of speech for this particular word is a noun.
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "explicit" is an adjective.
The part of speech for "twirl" is a verb.
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "lecturer" is a noun.