your explanation is totally wrong..
I've been doing this "not only..but also" rule since class 6th! they'r used together..

and speak is more formal when you don't know your subject personally and talk is when you know who your talking to.. since i'm not really good in explaining i'll paste the examples i digged out from google to make it easy on your head!..

"Both obviously relate to the business of communication. Broadly speak has to do with the use of oral language and talk has to do with oral language in relation to one or other person - even to oneslf.

You can speak a foreign language, speak to a group of people, speak the truth.

You can talk to another person/other people in the sense of having a conversation, you can talk about a subject in the sense of having a view about something.

The two nouns give a sharper defintion - speech (speak) and talk.
A talk gives the idea of an informal discourse a speech describes a formal occasion as in a debate.

Imagine a situation when someone is giving a speech to an audience and in that audience two children are having a conversation, not listening to the speech and irritating the rest of the audience. This is how you would describe that:

While the minister was speaking at the meeting, two children were told to be quiet because they were talking to one another and making it difficult for people to concentrate."

"The Rule: In the not only . . . but also construction, the two items connected must be similar in kind.

So:

Wrong: He not only painted the "Annunciation" but also [painted] the "Mona Lisa."
Right: He painted not only the "Annunciation" but also the "Mona lisa"


So in the not only wala question there's a placement mistake and in the clerk wala question there "speak" would be replaced with "talk"

Hope it helps =)