"The ducks swim."
It's a relatively safe bet that stress will be on the first or the second-to-last syllable of a word in its basic form. Additional syllables because of grammar (such as anka -> ankorna) rarely change the stress, but there are notable exceptions. Among them are two-syllable words that end on an unstressed short -or in their singular form, but get the -or- stressed and long in the plural. Motor -> motorer and traktor -> traktorer.
There are groups of words that tend to behave similarly though. Verbs that end on -era are usually stressed on the second-to-last syllable. Words that begin on be- and för- are usually stressed on the following syllable. And so on. I'm afraid you'll just have to practice and get the hang of it that way. After all, the brain is an amazing pattern recognition machine.