https://www.grammarbook.com/grammar/adjAdv.asp - Adjective and Adverbs Grammar Rules - R.P.S.K.Senavirathna - ANU/EN/2108/P/53

Definitions

• An adjective is a word or set of words that modifies (i.e., describes) a noun or pronoun. Adjectives may come before the word they modify.

Examples:
That is a cute puppy.
She likes a high school senior.

Adjectives may also follow the word they modify:

Examples:
That puppy looks cute.
The technology is state-of-the-art.

• An adverb is a word or set of words that modifies verbs, adjectives, or other adverbs. Adverbs answer how, when, where, why, or to what extent—how often or how much (e.g., daily, completely).

Examples:
He speaks slowly (tells how)
He speaks very slowly (the adverb very tells how slowly)
She arrived today (tells when)
She will arrive in an hour (this adverb phrase tells when)
Let's go outside (tells where)
We looked in the basement (this adverb phrase tells where)
Bernie left to avoid trouble (this adverb phrase tells why)
Jorge works out strenuously (tells to what extent)
Jorge works out whenever possible (this adverb phrase tells to what extent)

Rule 1. Many adverbs end in -ly, but many do not. Generally, if a word can have -ly added to its adjective form, place it there to form an adverb.

Examples:
She thinks quick/quickly.
How does she think? Quickly.

She is a quick/quickly thinker.
Quick is an adjective describing thinker, so no -ly is attached.

She thinks fast/fastly.
Fast answers the question how, so it is an adverb. But fast never has -ly attached to it.

We performed bad/badly.
Badly describes how we performed, so -ly is added.

Rule 2. Adverbs that answer the question how sometimes cause grammatical problems. It can be a challenge to determine if -ly should be attached. Avoid the trap of -ly with linking verbs such as taste, smell, look, feel, which pertain to the senses. Adverbs are often misplaced in such sentences, which require adjectives instead.

Examples:
Roses smell sweet/sweetly.
Do the roses actively smell with noses? No; in this case, smell is a linking verb—which requires an adjective to modify roses—so no -ly.

The woman looked angry/angrily to us.
Did the woman look with her eyes, or are we describing her appearance? We are describing her appearance (she appeared angry), so no -ly.

The woman looked angry/angrily at the paint splotches.
Here the woman actively looked (used her eyes), so the -ly is added.

She feels bad/badly about the news.
She is not feeling with fingers, so no -ly.

Rule 3. The word good is an adjective, whose adverb equivalent is well.

Examples:
You did a good job.
Good describes the job.

You did the job well.
Well answers how.

You smell good today.
Good describes your fragrance, not how you smell with your nose, so using the adjective is correct.

You smell well for someone with a cold.
You are actively smelling with your nose here, so use the adverb.

Rule 4. The word well can be an adjective, too. When referring to health, we often use well rather than good.

Examples:
You do not look well today.
I don't feel well, either.

Rule 5. Adjectives come in three forms, also called degrees. An adjective in its normal or usual form is called a positive degree adjective. There are also the comparative and superlative degrees, which are used for comparison, as in the following examples:

Positive	Comparative	Superlative
sweet	sweeter	sweetest
bad	worse	worst
efficient	more efficient	most efficient

A common error in using adjectives and adverbs arises from using the wrong form of comparison. To compare two things, always use a comparative adjective:

Example: She is the cleverer of the two women (never cleverest)

The word cleverest is what is called the superlative form of clever. Use it only when comparing three or more things:

Example: She is the cleverest of them all.

Incorrect: Chocolate or vanilla: which do you like best?

Correct: Chocolate or vanilla: which do you like better?

Rule 6. There are also three degrees of adverbs. In formal usage, do not drop the -ly from an adverb when using the comparative form.

Incorrect: She spoke quicker than he did.

Correct: She spoke more quickly than he did.

Incorrect: Talk quieter.

Correct: Talk more quietly.

Rule 7. When this, that, these, and those are followed by a noun, they are adjectives. When they appear without a noun following them, they are pronouns.

Examples:
This house is for sale.
This is an adjective.

This is for sale.
This is a pronoun.