We use not only X but also Y in formal contexts:

The war caused not only destruction and death but also generations of hatred between the two communities.

The car not only is economical but also feels good to drive.

This investigation is not only one that is continuing and worldwide but also one that we expect to continue for quite some time.

We can sometimes leave out also:

I identified with Denzel Washington not only as an actor but as a person.

To add emphasis, we can use not only at the beginning of a clause. When we do this, we invert the subject and the verb:

Not only was it raining all day at the wedding but also the band was late.

Not only will they paint the outside of the house but also the inside.

When there is no auxiliary verb or main verb be, we use do, does, did:

Not only did she forget my birthday, but she also didn’t even apologise for forgetting it.

See also:

Inversion