"The ducks swim."
Could anyone explain me why there is a stress on the first syllable of "ankorna"?
Swedish has somewhat irregular stress. Anka is stressed on the first syllable, and it stays this way regardles of the fact that a syllable is added to make it plural definite.
So, this means there are no rules regulating the stress and intonation patterns. Is there a way for me to make as few mistakes as possible while trying to put stress on a new word then?
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.