Zij plakt een slinger op de muur.

In Dutch, zij is the full subject pronoun for “she.” You often see zij
• in written or formal contexts,
• when you want to add emphasis (“She, and not someone else, is doing it”).

In everyday speech, native speakers frequently shorten zij to ze: “Ze plakt een slinger op de muur.”

Plakken is a regular verb. In the present tense, you use the stem plak- plus -t for the third person singular:
• ik plak
• jij plakt
• hij/zij/het plakt

So zij plakt is simply “she sticks/glues.”

• een = “a/an,” the indefinite article.
• You use een because we’ve introduced slinger for the first (and only) time; we don’t assume the listener knows which specific one.
If you had already mentioned or both speaker and listener know the specific streamer, you’d say de slinger (“the streamer”).

• slinger = “streamer” or “garland” (a decorative hanging ribbon).
• It’s a de-word (common gender): de slinger.

In Dutch, op generally means “on (top of a surface).”
• You say een schilderij ophangen aan de muur for “hang a painting on the wall,” because aan suggests attachment to a vertical surface by hook or nail.
• With plakken (sticking with glue/tape), you use op: plakken op de muur = “stick on the wall.”

Dutch typically places the direct object (een slinger) immediately after the finite verb (plakt). The prepositional phrase (op de muur) comes afterward as a location complement. Swapping them sounds odd to a native speaker.