Anna hangt slingers in de feestzaal.

In Dutch, the third-person singular present tense of most verbs adds -t to the verb stem. Here the infinitive hangen has the stem hang-, so for zij (she) or Anna you get zij hangt. The present-tense paradigm looks like this: • ik hang
• jij/u hang(t)
• hij/zij/het hangt
• wij/jullie/zij hangen

• hangen (intransitive or transitive) simply means “to hang” or “to be hanging.”
Example: Anna hangt slingers in de feestzaal.
• ophangen (separable: hangt op, hingen op) means “to hang up” (put something up).
Example without location: Anna hangt de slingers op.
With location at the end: Anna hangt de slingers in de feestzaal op.
If you front the location: In de feestzaal hangt Anna de slingers op.

Because slingers is an indefinite plural. In Dutch: • Indefinite plural (some/—): no article → Anna hangt slingers…
• Definite plural (the): use de → Anna hangt de slingers… (if you mean specific ones)
• Indefinite singular (one): use een → Anna hangt een slinger…

Although there are exceptions, here are broad guidelines:
• Use -en for most monosyllabic nouns and those with stress on the last syllable (e.g., boek → boeken, raam → ramen).
• Use -s for many polysyllabic nouns ending in a light or unstressed syllable (especially those ending in -er, -el, -em, vowel) (e.g., tafel → tafels, slinger → slingers, café → cafés).
Always check a dictionary for irregulars and exceptions.