Really now ...

 In answer to a question shared on my Facebook page yesterday regarding whether "really" is an adverb in the sentence "Peter is really tired":

In “Peter is really tired” the word “tired” is undoubtedly an adjective. A simple test is to see whether words which we know without a doubt to be adjectives can be put in its place – which they can, e.g. Peter is handsome; Peter is sick. Placing “really” before “tired” doesn’t alter what part of speech “tired” is.

Where could the idea come from that “tired” may be a verb (since it wouldn’t intuitively be taken to be one)? The answer is that there are many instances where the so-called past participle of a verb (but NB: only when it occurs in a passive voice sentence) can function as, or be confused with, an adjective. E.g. the sentence “Peter was puzzled” could be understood as an abbreviated passive voice version of, say, “Peter was puzzled by the sudden change in weather”, of which the active voice version would be “The sudden change in weather puzzled Peter.” A passive voice interpretation of “Peter was puzzled” would be rare in normal, everyday speech, though. Our natural interpretation of “puzzled” in “Peter was puzzled” would be to understand it as an adjective. Ditto, then for “tired” in “Peter is tired.”  

So is “really” an adverb? Traditionally an adverb is a part of speech that is said to “modify” a verb (i.e. “tell us more about” the action named by a verb), or another adverb, or an adjective. Since “tired” is an adjective, “really” is an adverb in this case. Again, the substitution test can help us confirm this: “very” and “quite” are two words whose adverbial status is traditionally taken for granted, and since both of them can replace “really” here (cf.  Peter is quite tired; Peter is very tired) we can assume that “really” is an adverb. Add to this the fact that the suffix “-ly” is usually indicative of an adverb, and there can be little doubt about the adverbial status of “really” in "Peter is really tired."