Adverbially adjectival: GOOD vs. WELL

Many adjectives have an adverbial form produced by adding an -ly ending:

• quick → quickly
• accidental → accidentally
• local → locally
• recent → recently
• frequent → frequently
• happy → happily [replace -y with -ily]

Other adjectives have an irregular adverbial form:

• good → well [instead of goodly]
• early → early [adjective already ends in -ly]

Modern theories of grammar include adverbial forms of adjectives within the class of adjectives much as singular and plural nouns are included within the class of nouns. The reason is because they share most of the behaviour of the adjectives they are derived from. For example, gradable adjectives and adverbs both behave the same way in comparisons:
COMPARISONS WITH ADJECTIVES:

• Jill is as efficient as Penny.
• Jill is less/more efficient than Penny.
• Jill is the least/most efficient student.

COMPARISONS WITH ADVERBS:

• Jill studies as efficiently as Penny.
• Jill studies less/more efficiently than Penny.
• Jill studies the least/most efficiently.

Both can also be modified with degree words such as very, too, so, quite, really, pretty, rather, etc.:

• Jill is very/too/so/quite/really/pretty/rather efficient. [adjective]
• Jill studies very/too/so/quite/really/pretty/rather efficiently. [adverb]

Languages like German don't have different forms for the regular and adverbial forms of adjectives, which makes it difficult for German speakers to learn when to use each in English. The most confusing examples are sentences like these:

• Jill performed well. [adverb]
• Jill looked good. [adjective]

So why is it performed well (an adverb), but looked good (an adjective)?
It helps to recognise that the sentence with perform is about an activity, while the sentence with look is about having the particular state or quality described by the adjective. The first says that Jill did something good and the second says that Jill was something good. The second sentence is not saying that Jill was good at looking at things. It means that her appearance was good. Look in this sense (not to be confused with the other meanings of the word) is a stative verb as are the small number of other verbs that select adjectival complements. These include seeming verbs such as seem, appear, look, sound, feel, smell and taste, as well as verbs such as be, become, stay and remain.

• Jill was good.
• Jill seemed good.
• The photograph looked good.
• The music sounded good.
• The material felt good.
• The perfume smelled good.
• The cake tasted good.

The difference between good and well is further complicated by the fact that there is also an adjective well, which means in good health, so it is also possible to say:

• Jill was/seemed/looked/sounded/felt well. [the adjective well]

Another source of confusion involves hard and hardly in examples like these:

• Jill worked hard. [worked a lot]
• Jill hardly worked. [worked only a little]

When hard is used with the meaning of either not soft or difficult, it's a regular adjective, but here (despite not ending in -ly), it's an adverb meaning with great effort. Hardly is also an adverb, but it has almost the opposite meaning. It means almost not at all:

• Jill hardly had any time to work today because the phone kept ringing. [she had almost no time]

So it's not a good idea to thank people for hardly working, but it makes perfect sense to thank them for working hard.
Advanced
ADJECTIVES
Adjectives can be used to modify nouns, but are also used in a variety of more exotic constructions. Here are some examples:

• Jill had a sick child. [an 'attributive' adjective modifying the noun child]
• Jill was sick. [complement of the verb be (a 'predicative' adjective)]
• Jill felt sick. [complement of the verb feel]
• Jill was worrying herself sick. [a 'resultative' construction]
• There was something unusual about Jill. [a 'postpositive' modifier of the noun something]

ADVERBS AND ADVERBIALS
Phrases with an adverbial function can often be identified by asking where, when and how questions.

• A: Where did Jill perform?
• B: She performed on Broadway/in New York/here/there. [place]
• A: When did Jill perform?
• B: She performed yesterday/today/at five o'clock. [time]
• A: When does Jill perform?
• B: She performs every day/often/on Mondays. [frequency]
• A: How did Jill perform?
• B: She performed well. [manner]

But this rule is unreliable in a lot of cases. For example, with the seeming verbs discussed earlier, the how question can be answered with a regular non-adverbial adjective:

• A: How did Jill look?
• B: Jill looked good. [adjective]

Another sub-category of adverbs includes words for places such as here and there, and times such as now and then. These words can appear in the same positions within a sentence as phrases headed by prepositions such as in the bedroom (a place) and at five o'clock (a time), so just as we grouped adverbial adjectives with adjectives, we can group these adverbs with preposition phrases. Indeed, modern approaches to grammar do not treat adverbs as a distinct part of speech at all. There are only adverbial functions, which can be fulfilled by phrases of various types.
Degree words are yet another category traditionally included within the class of adverbs. Degree words modify gradable adjectives.
The traditional category of adverbs includes words that can modify verbs, adjectives, other adverbs, prepositions and whole clauses. Here are some examples:

• Jill performed well. [modifying the verb perform]
• Jill was very slow. [modifying the adjective slow]
• Jill walked very slowly. [modifying the adverb slowly]
• Jill walked right over me. [modifying the preposition over]
• Naturally, Jill spoke English. [modifying the clause Jill spoke English]