Han trykker på knappen.

In Norwegian the verb trykke needs the preposition på when you “press on” something. You can treat trykke på as a single unit (a phrasal verb) meaning to press.
• In English you say press a button, but in Norwegian you always say trykker på knappen.
• Never say trykker knappen—that is ungrammatical.
• You’ll see the same pattern with other verbs: vente på (wait for), kikke på (take a look at), satse på (rely on).

knappen is the definite singular of knapp (button). Norwegian nouns have these forms:
• Indefinite singular: en knapp
• Definite singular: knappen
Here you’re referring to a specific, known button—hence the definite form knappen.

Use the indefinite form en knapp:
Han trykker på en knapp.
This means “He presses a button” (any button), whereas Han trykker på knappen means “He presses the (specific) button.”

trykke is a weak (regular) verb. Here are key forms:
• Present: jeg trykker, du trykker, han/hun trykker, vi trykker, dere trykker, de trykker
• Simple past: jeg trykket, … de trykket
• Present perfect: har trykket
• Future: vil trykke or skal trykke

IPA: /ˈtrykːər/
Tips:
• y is like the French u in lune (a front rounded vowel).
• kk signals a long or doubled /k/ sound.
• The ending -er sounds like [ər], with a quick, reduced vowel.

Basic word order in a main clause is Subject–Verb–Object/Complement. Here:
Han (subject) trykker (verb) på knappen (object).
If you move på knappen to the front, you must invert verb and subject (the V2 rule):
På knappen trykker han.