Learn Dutch: We hangen de ballon stilletjes op, zodat we hem kunnen verrassen. - We are quietly hanging up the balloon so that we can surprise him.

While stil just means "quiet" or "still", adding -etjes creates an adverb of manner (stilletjes) that translates to "quietly", "secretly", or "on the sly". It adds a softer, more deliberate nuance to the action, which fits perfectly when sneaking around to set up a surprise.

Why is op placed at the very end of the first clause, after stilletjes?

The verb is ophangen (to hang up), which is a separable verb. In a main clause, the conjugated part (hangen) takes the second position, while the prefix (op) gets kicked to the very end of that clause. The adverb stilletjes must come before this final prefix.

Why do both verbs (kunnen verrassen) go at the very end of the second clause?

The conjunction zodat (so that) introduces a subordinate clause. In Dutch subordinate clauses, all verbs are pushed to the end. When you have a modal verb (kunnen) and an infinitive (verrassen), they group together at the end of the sentence.

Could we use an om ... te construction here instead of zodat?