An adjective is a part of speech that modifies a noun or pronoun.  Adjectives usually tell what kind, how many, or which about nouns or pronouns.

An adverb is a part of speech that modifies a another adverb, a verb, or an adjective. It is often recognized by the suffix -ly at the end of it. Adjectives usually describe an action in terms of how, when, where, and to what extent it occurred.

To avoid errors, identify what word the adverb or adjective in question modifies.

• noun or a pronoun -> use an adjective
• verb or another adverb -> use an adverb

Sometimes, the use of a certain adverb requires the inversion of the subject and the verb. If a sentence begins with a negative adverb or an adverb with restrictive meaning, it must have an inverted word order.

• Nowhere else have I seen such a talented artist.
• In no circumstances should we allow this behavior to continue.

Some other adverbs with restrictive meaning that require inversion of the verb and subject are:

• hardly ever
• hardly... when
• neither/nor
• never before
• no sooner... than
• not only
• seldom

Bad or Badly

Bad is an adjective used with linking verbs such as feel, seem, be, look, etc.

• I feel bad that she lost her phone.

Badly is an adverb used to modify action verbs.

• Sometimes job interviews go badly.

 

Calm or Calmly

Calm is an adjective, and it is used with linking verbs to modify nouns and pronouns.

• She appeared calm after she heard the news.

Calmly is an adverb that modifies verbs.

• She approached the stray dog calmly.

 

Easy or Easily

Easy is an adjective used with linking verbs to modify nouns and pronouns.

• This assignment did not look easy.

Easily is an adverb, and it is used to modify verbs.

• The couple was moving easily around the dancefloor.

 

Good or Well

Good is an adjective. It is also often used with linking verbs.

• It felt good to win the championship.

Well, when used as an adjective, implies "in good health."

• My grandmother looks well even now in her nineties. 

Well, when used as an adverb, well means "expertly."

• My friend plays the guitar well.

 

Real or Really

Really is an adverb, and it modifies other adverbs, verbs, or adjectives. It has a meaning of "very."

• He did really well on his performance review.

Real is an adjective, and can be used to modify nouns or noun phrases. It has a meaning of "true or genuine."

• She was a real genius when it came to spreadsheets.

 

Slow or Slowly

Slow can be used as an adjective.

• This is a slow train.

Slow can also be used as an adverb. 

• The traffic is moving slow.

Slowly is an adverb. It can replace slow anywhere it is used as an adverb. Slowly also appears in sentences with auxiliary verbs where slow cannot be used.

• He has been slowly recovering from the accident.