Well, we use it when we have two things and we want to give a little extra emphasis to the second thing because it’s even better, or even worse, or more surprising, or more impressive, or more shocking than the first thing.

There are two ways to use not only, but also: 
You can use it in the middle and end of the sentence, or you can actually use it to start a sentence with not only.
There are some grammatical considerations we have to keep in mind.

Note: 
Not only, but also can be used in the middle and end of a sentence.

Example: I’ve attended not only Public Speaking Sessions but also interview techniques at Wabs Talk.

Example: Mr.Reuben Singh is not only diligent but also flamboyant.

As you can see in both of the above examples, the second part, the part that comes after “but also” is just a little more interesting or better than the first part, so we just want to give a little extra emphasis.

Note:
If you use an adjective after not only, you need to use an adjective after but also.

Incorrect: She’s not only beautiful, but also a singer.
The above sentence has an error because beautiful is an adjective, and a singer is a noun, and it’s not parallel.

Correct: She’s not only a model, but also a singer.
You could have two nouns.That sounds natural, because we have a noun and a

Incorrect: He ate not only the pizza, but also the soda.
That doesn’t make sense because you eat pizza, but you don’t eat soda. You drink soda.

Correct: He not only ate the pizza, but also drank the soda.

Note: 
We can fix this sentence by using two different verbs after not only and but also.

How about using ‘not only’ to start a sentence?

Note:
When you start it with not only, you have to use auxillary verb ‘does’ .

Example: Not only does he attends the weekend workshops, he also participates in outdoor and indoor competitions.

How about using not only in the past?

If we’re talking about the past, you can also use this construction, but now you need to use the auxiliary verb, ‘did’ in the past.

Example: Not only did she cracked the interview, but she also got admiration for her skills.

Note:
After but also, just use the simple past. You don’t need to use an auxiliary verb there.

You can also use this construction in the future.

Example: Not only will you learn grammar in my advanced English grammar course, you’ll also put it into practice.” In this case, the auxiliary verb is ‘will’.