[1] The only way to do it is very, very slowly. [complement of specifying be]

There are constraints on manner adverbs as complement of "be", but there are one or two, such as your example above, and one or two others, such as:

[2] It was only very reluctantly that she agreed.

Slowly" is a manner adverb; it describes how, in what way, the process in the verb phrase is performed. Adjectives don't do that; essentially, they describe nouns not verbs, so the adjective "slow" would be quite wrong in [1].

"Tomorrow" is not an adverb but an NP with the deictic pronoun "tomorrow" as head functioning as an adjunct of temporal location. No: you can't say *You are so beautifully" "Beautifully" is an adverb, so it cannot be a subject complement of "be" describing "you".

Note that there is an adverb "slow" that is formed from the adjective "slow" by conversion (as opposed to by -ly suffixation), as in "He was walking quite slow because of his injury", but this use is condemned by prescriptivists

[1] The only way to do it is very, very slowly. [complement of specifying be]

There are constraints on manner adverbs as complement of "be", but there are one or two, such as your example above, and

[2] It was only very reluctantly that she agreed.

Slowly" is a manner adverb; it describes how, in what way, the process in the verb phrase is performed. Adjectives don't do that; essentially, they describe nouns not verbs, so the adjective "slow" would be quite wrong in [1].

"Tomorrow" is not an adverb but an NP with the deictic pronoun "tomorrow" as head functioning as an adjunct of temporal location. No: you can't say *You are so beautifully" "Beautifully" is an adverb, so it cannot be a subject complement of "be" describing "you".

Note that there is an adverb "slow" that is formed from the adjective "slow" by conversion (as opposed to by -ly suffixation), as in "He was walking quite slow because of his injury", but this use is condemned by prescriptivists

[1] The only way to do it is very, very slowly. [complement of specifying be]

There are constraints on manner adverbs as complement of "be", but there are one or two, such as your example above, and one or two others, such as:

[2] It was only very reluctantly that she agreed.

Slowly" is a manner adverb; it describes the nature of the action. Adjectives don't do that; they describe nouns not verbs, so the adjective "slow" would be quite wrong in [1].

"Tomorrow" is not an adverb but an NP with the deictic pronoun "tomorrow" as head functioning as an adjunct of temporal location. No: you can't say *You are so beautifully" "Beautifully" is an adverb, so it cannot be a subject complement of "be" describing "you".

Note that there is an adverb "slow" that is formed from the adjective "slow" by conversion (as opposed to by -ly suffixation), as in "He was walking quite slow because of his injury", but this use is condemned by prescriptivists

[1] The only way to do it is very, very slowly. [complement of specifying be]

There are constraints on manner adverbs as complement of "be", but there are one or two, such as your example above, and one or two others, such as:

[2] It was only very reluctantly that she agreed.

Slowly" is a manner adverb; it describes how, in what way, the process in the verb phrase is performed. Adjectives don't do that; essentially, they describe nouns not verbs, so the adjective "slow" would be quite wrong in [1].

"Tomorrow" is not an adverb but an NP with the deictic pronoun "tomorrow" as head functioning as an adjunct of temporal location. No: you can't say *You are so beautifully" "Beautifully" is an adverb, so it cannot be a subject complement of "be" describing "you".

Note that there is an adverb "slow" that is formed from the adjective "slow" by conversion (as opposed to by -ly suffixation), as in "He was walking quite slow because of his injury", but this use is condemned by prescriptivists