Learn Swedish: Vi ställde tillbaka sakerna på hyllan. - We put back the things on the shelf.

No, you cannot. In Swedish, the particle (tillbaka) must come directly after the verb (ställde) and before the object (sakerna). English lets you separate them to say "put the things back", but Swedish requires the verb and particle to stay together here.

Where does the stress fall when pronouncing ställde tillbaka?

In Swedish particle verbs, the stress always falls heavily on the particle, not the verb. You should emphasize tillbaka and pronounce the verb ställde quite quickly and lightly in comparison.

Why does this sentence use ställde for "put"?

Swedish lacks a single generic word for "put"; you must specify the object's physical position. ställde is the past tense of ställa, which means to put something in a standing or upright position, like books on a shelf. If you were putting down objects that lie flat, you would use lade tillbaka instead.

What is the base form of sakerna?

The base noun is en sak (a thing). The plural is saker (things), and to say "the things", you add the definite plural ending -na to make sakerna.