But advertisements do not only give information, they also try to influence our emotions.
In this sentence, I'd like to know the usage of "do".

Let's take this slowly, so you see the whole picture.

Write these three statements as questions:

(1) I can come to your party.

(2) I have finished my homework.

(3) They eat cereal for breakfast.

Dear Claire,
I don't understand what you mean.

"do" is for emphasis. Omit it, and the meaning of the sentence remains unchanged.

Imaginary conversation: "But doctor, I do take my meds every day w/o fail."
Delete the "do" and the sentence meaning remains unchanged, only the emphasis is reduced.

What don't you understand? I'll do one for you:
Statement: I have written a letter to him.
As a question: Have you written a letter to him?

I want to show you something about what happens, before I show you the same thing in your sentence.

And to Misha: No, Misha, NO. That is incorrect.
I'll teach you English grammar AFTER I've answered the question in progress, which - just like an American - you have so IMPOLITELY interrupted. Have you Americans no manners whatsoever? You just override someone and take over!

Dear Claire,
Do you mean that "do" is the marker for a negative simple present tense verb in the sentence I mentioned above? Please continue to explain it to me.

Dear Claire, I overrode nothing. The subject lay fallow for several hours and I added an additional explanation for the requestor since he obviously didn't understand your answer, poorly worded as it was. I also doubt your ability to teach me US English grammar, but I ascribe this shortcoming to your lack of experience and haughty attitude to absorb any comment you disagree with or any challenge to your falsely assumed absolute authority in these matters. No one has that authority.

Hello thein lwin

"Do" is a dummy auxiliary verb in your example. It has no meaning of its own - it simply permits compliance with the grammatical requirement that negative clauses should under certain conditions contain an auxiliary verb. Which is why it's referred to as the dummy auxiliary do.

But in sentences with negative + positive coordination (as in your example), where "not only" precedes a lexical (i.e. non-auxiliary) verb like "give", then "do" can be omitted, so we get alternation between the two constructions:

1. But advertisements do not only give information, they also try to influence our emotions.

2. But advertisements not only give information, they also try to influence our emotions.

In ex 1. (yours) syntactically the negation is associated with "do" as opposed to "not only" in ex 2. where "do" has been omitted".

Is that what you wanted to know?

PaulM