knöpfen
to button
To close a garment or something similar with buttons. The opposite of 'aufknöpfen' (to unbutton).
Kannst du mir bitte das Hemd am Kragen knöpfen?
Can you please button my shirt at the collar?

to button
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to button
To close a garment or something similar with buttons. The opposite of 'aufknöpfen' (to unbutton).
Kannst du mir bitte das Hemd am Kragen knöpfen?
Can you please button my shirt at the collar?

The verb 'knöpfen' (to button) is derived directly from the noun 'der Knopf' (button). It describes the action you perform with a button.
Das Kind lernt gerade, seine Jacke selbst zu knöpfen.
The child is just learning to button his jacket by himself.
In everyday life, the separable verbs 'zuknöpfen' (to button up/close) and 'aufknöpfen' (to unbutton/open) are often used. They are more precise than 'knöpfen' (to button) on its own.
Es ist kalt, du solltest deine Jacke zuknöpfen.
It's cold; you should button up your jacket.
The idiom 'sich jemanden vorknöpfen' (to take someone to task) has a fixed structure. The person acting is the subject, the reflexive pronoun 'sich' is obligatory, and the person being criticized is in the accusative.
Der Chef hat sich den neuen Mitarbeiter vorgeknöpft.
The boss took the new employee to task.
This figurative meaning is easy to recognize. It is always used with the reflexive pronoun 'sich' and the separable prefix 'vor-'.
Ich werde mir morgen den Verkäufer vorknöpfen, der mir das kaputte Gerät verkauft hat.
Tomorrow I'm going to take the salesperson to task who sold me the broken device.
to open
'öffnen' and 'knöpfen' can go in opposite directions when it comes to clothing. 'öffnen' generally means to open something that is closed. With a jacket or a shirt, you can say that you open it when you undo it. 'knöpfen', on the other hand, means to close a garment with buttons. It is much more specific than 'öffnen' because it is only about buttons. The direct opposite of 'knöpfen' is actually 'aufknöpfen' (to unbutton), but in the sense of opening something, 'öffnen' is also an opposite.

Vor dem Bewerbungsgespräch knöpft Mehmet sein Hemd sorgfältig.
Before the job interview, Mehmet carefully buttons his shirt.
Im warmen Zug öffnet Paul seine Jacke.
On the warm train, Paul opens his jacket.