If it ends in "ly" it's an adverb. Of course, not all adverbs end in "ly" so if you are in doubt, substitute another "ly" word in the sentence and see if it "works."
I live in a big house.
What is "big"? If it's an adverb, you can substitute some "ly" word - like, maybe "quickly" or maybe "immensely" into the sentence:
I live in a quickly house.
I live in an immensely house.
Sorry - doesn't work!!
So "big" must NOT an adverb. In fact, it is an adjective, because adjectives modify NOUNS (and "house" is a noun).
Adverbs modify (that is, apply to, or describe) verbs or clauses.
I ran to the store quickly.
"Quickly" tells us how you ran. You can't say, "I ran to the store big."
You CAN say, "I ran to the BIG store quickly" however. In that case, the "ly" word refers to the verb ("ran") and the big refers to the noun ("store").
The trick is in words that can be either, adverb or adjective, so in this case you have to think which word they really are "modifying" or referring to:
It was a very big store.
In this case "very" can either modify "big" or "store" - but if you take away the "big" and try the sentence as "It was a very store" - it doesn't make sense. Therefore, "very" must be talking about how big. It was "very" big. So, very is modifying NOT a noun (store) but another adjective ("big") so ... it's an adverb.
Try substituting a "ly" word for "very" and you'll see:
It was an immensely big store.
See .. the "ly" tells us it is an adverb!
That word, "very", can indeed be an adjective:
It was the very house we were looking for.
In that case, "very" has to be modifying or referring to "house" and since "house" is a noun, it must be an adjective.
If you can tell the difference between a NOUN and anything else, then you only have to memorize one more rule: if the word modifies ("refers to") the NOUN, then it must be an adjective. Othewise, it's not! And if the question is simply if it's an Adj, or an Adv, then it must be an Adv.
The debate that will rage forever (oh, the pedants among us will think they have it solved ... but believe me, the debate rages) is in sentence like
"I feel good"
I won't go there (but I bet you someone ELSE will!!)
Please DO remember however that "well" is an adverb and "good" is an adjective in sentence of less debatable construction:
He can speak English well. (or, He speaks well.)
He speaks good English.
He writes his letters well. (or, He writes well.)
He writes good letters.
Now, if you have learned anything from my posting, can you tell me WHY each of the four examples above is correct, and what each adjective or adverb is modifying?
Student or Learner