Not Only But Also Rules

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Subject – Verb Agreement

Whether to use a singular verb or a plural one depends on the second part (but...also / as well). See subject verb agreement in general.

For Example:

Not only John but also Peter ---- come to the meeting.
A) have   B) has

As "Peter" is singular, we need to use a singular verb, so the answer is "has".

Pay attention to the sentences below to understand the difference clearly.

Not only proteins but also vitamins are essential for small children.
Note that the second part "vitamins" is plural.

Not only proteins but also vitamin C is essential for small children.
Note that the second part "vitamin C" is singular.

Attention!

Both proteins and vitamin C are essential for small children.
Don't confuse "Both...and" with "Not only...but also" since it is always used in plural form.

Not Only...But Also Inversion

Inversion refers to an irregular sentence order and is used to emphasize the event usually beginning with a negative.

She not only apologized but also sent me a card.
Not only did she apologize but (also) sent me a card.

Not only do I enjoy spending time with him, but I also feel safe with him.

Note:

We can use “as well” instead of “also”, however, we use it at the end of the sentence.

This place not only depressed me but made me want to go home as well. ↑▲▲▲▲▲▲▲↑