Look at these sentences taken from an article about healthy eating habits.
A side salad is the healthier option. Not only does it provide plenty of vitamin C, it also contains loads of fibre.
The salad mentioned has two qualities: (1) it provides plenty of vitamin C, and (2) it contains loads of fibre. By putting not only at the front of the sentence, and also before the second quality, we emphasise the fact that the second quality also exists.
Notice the word order after not only:
Not only + auxiliary + subject (+ main verb)
Not only does it provide…
Notice that also comes after the subject and before the main verb:
…it also contains loads of fibre.
In the case of the verb to be and auxiliary verbs, also comes after:
…it's also good for you.
…it has also rained.
1. Inversion after not only.
Write in the gaps to complete a sentence with not only and also.
For more information about inversion after not only see the Grammar Reference.