Which one/ones of below may this sentence mean in context?
1) "I chose to study Swedish today rather than going out." 2) "I chose to study Swedish language as my major." 3) "I chose to study Swedish so I constantly practice with the intention of mastering it at the end."
Why do we must use "att" in some places and in some it is not neccsary ?
I think there are just some verbs which negate the need for the 'att', and you just have to learn them. I don't think there are that many, but they are quite well-used.
Modals and verbs that behave like modals never take the att. The general rule is that all others do. However, there are quite a few of the latter for which the att is optional, and those you do unfortunately have to learn one by one.
I decided to study Swedish -- What's wrong with that? Why was it rejected?
That's not really idiomatic English, though. And "select" is generally better as välja ut in Swedish, in the sense of picking one out of many.