When you add sugar to it, your cup of coffee becomes not only sweet, also flattening.

Which word is not grammatically used in this sentence ?

What does the sweetened coffee flatten? The word is not the correct word.

......your cup of coffee not only becomes sweet, but also flattening.

or
.......your cup of coffee becomes not only sweet, but also flattening.

To identify which word is not grammatically used in the sentence, we will need to examine each word and its usage. Here's a breakdown of the sentence:

"When you add sugar to it, your cup of coffee becomes not only sweet, also flattening."

In this sentence, the word "also" is not used correctly. "Also" is an adverb that is used to add information to a sentence, but it is typically placed before the verb phrase it modifies. However, in this sentence, "also" is placed after the verb "becomes," which creates an awkward and incorrect construction.

To correct the sentence, we can rephrase it as follows: "When you add sugar to it, your cup of coffee becomes not only sweet but also flat."

In this revised version, the word "also" is placed correctly before the adjective "flat," making the sentence grammatically sound.

The words after the comma should be "but fattening."