He could not only understand it but also I could review the problem again.

I'm Korean. My teacher said two subjects can not be used in "not only... but also..." sentence so he said my sentence is wrong, but I can't understand why my sentence is grammatically wrong.

  • Not only could he understand it, but I could also review the problem again is grammatical and understandable. As such, it's quite possible to use the construction you have correctly—just not in exactly the way that it's posed in the question. As such, it's not clear what you're actually asking. Nov 9, 2019 at 13:57

3 Answers 3


It is wrong.

He could not only understand it but also I could review the problem again.

When you use "not only... but also..." it is like you are making two 'branches' of the same sentence. What appears after "not only" and after "but also" should be able to make a complete sentence with what went before, for example:

I ate not only my sandwich but also my fries.


  • I ate my sandwich
  • I ate my fries

The problem with your sentence is that the first part is about someone else, but the second part about you. You are in effect saying:

  • He could understand it.
  • He could I could review the problem again <-- ungrammatical

You should instead say either:

Not only could he understand it, but also I could review the problem again.

Or, a much simpler way to express this would be:

He could understand it, and I could review the problem again.

  • 1
    Your first correction sounds a lot more natural to me as "but I could also" &mdash; there's some weird rules on where the "also" should go, I feel; to my ear it should go after the subject and also after any auxiliary or modal verbs (but not after the main verb).
    – Muzer
    Nov 8, 2019 at 18:59

Your teacher is right that some complex sentence should not have two different subjects. Subject of one part of the sentence is "he", subject of the other is "I", and that is confusing.

On my first reading, I understood it as:

He could not only understand it but also [he] could review the problem again.

which shows the confusion.


As formatted in your original sentence, it contains two complete sentences with Subject/Verb/Object.

I believe if you placed a semicolon (;) before the word but in your original format, it would be grammatically valid.

You are writing the way you hear. This is usually a safe procedure. But in this case, you need a special punctuation mark to have it comply with general grammatical rules of sentence structure.

  • This is incorrect. not only... but also must all be part of the same sentence/clause. Also, any clauses separated by a semicolon must be able to stand as complete sentences on their own. Neither He could not only understand it nor But also I could review the problem again are grammatical sentences. Nov 8, 2019 at 19:36
  • @GentlePurpleRain By removing the phrase, “but also”, both can stand as independent sentences. That is why I speculated that a semicolon would perhaps make it grammatically valid. It wouldn’t be elegant, but valid. “He could not only understand it.” means his understanding wasn’t limited to ’it’. He had understanding of other things too. Nov 8, 2019 at 20:09
  • I think the meaning you're after would be better stated as "He could not understand only it". Regardless, by using this interpretation and removing the "but also", you're completely changing the meaning of the sentence, which doesn't help the OP at all. Nov 8, 2019 at 20:23
  • @GentlePurpleRain The question from the OP is based upon the statement from the teacher that two subjects cannot be used in a sentence. That is not correct. Nov 8, 2019 at 20:25
  • The statement was that two subjects cannot be used in a "not only... but also" sentence, which your proposed modifications have annihilated. Obviously a conjunction of two clauses using a semicolon can have two subjects, but then it is no longer a "not only... but also" sentence. Nov 8, 2019 at 20:27

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