Sentence with doubtful syntax

I believe my first impression could be justified (problem). After having thought about the several comments to the question, it finally occurred to me that "not only… but also" was a unit that could leave some trace somewhere and the check I made was conclusive.

(Grammarly: Parallelism with "not only…but also)
When using "not only . . . but also" in a sentence, parallelism should be the goal. It means that the words following both parts of this correlative conjunction (i.e., not only and but also) should belong to the same parts of speech. For example, if a verb follows "not only", then a verb should also follow "but also". Using different parts of speech after each part of "not only . . . but also" makes the sentence imbalanced and, frankly, just awkward.

A slight change according to this principle, which results in the sentence below, would appear to give back to the initial sentence a normal feel, but that is without taking into account another principle, the verb/subject inversion in sentences beginning with "not only" (cambridge Grammar).

• Not only verbs are largely uninflected in English, but also nouns, pronouns and adjectives.

However, respecting the two principles is possible in the following way, the result being a satisfying, balanced sentence.

• In English, are largely uninflected not only verbs, but also nouns, pronouns and adjectives.

Another option, perhaps better

• In english, not only verbs but also nouns, pronouns and adjectives are largely uninflected.

The sentence structure is a bit awkward even if I ignore the grammatical error. Still, the main issue that I see is that you have but also preceding a dependent clause rather than an independent clause.

Perhaps something such as the following could work:

Verbs are not the only words that are largely uninflected in English. Additionally, nouns, pronouns, and adjectives are largely uninflected as well.

If you wanted a compound sentence then:

In addition to verbs, the English language has largely uninflected nouns, pronouns, and adjectives.

The underlying issue (the awkwardness that I spoke of) is that you are really going to have to be unnecessarily wordy to make this work. The best solution to limit wordiness would be this:

English is largely uninflected.

(I took the last one directly from the Oxford Languages Dictionary.)

Thanks to @Hellion for catching word usage mistakes.

Thanks to @LPH for catching that the compound sentence example is very weak. I have created a new example in its place.