Why is here added the e to "we wtorek" but in the other sentence in this lesson there wasn't it in "w więzieniu"? I understand that "e" makes it easier to pronounce but why isn't it used everytime two w's are in the row? Also why is it "we wtorek" instead of "we wtorku"? So why nominative not locative? :) Dzięki!
Using present tense because odwiedzać is a perfective verb? If it was imperfective, would it look like this: "One (oni) będą odwiedzawać/odwiedzawały (odwiedzawali) farmę [utorkiem]"?
How do I say that they visit regularly on Tuesdays, is it as I wrote (instrumental of the day noun)?
"odwiedzać" is imperfective. A perfective verb cannot be used in the Present Tense ;)
So "Oni odwiedzają farmę" is Present Tense and imperfective, "Oni odwiedzą farmę" is Future Tense, and perfective.
And imperfective in the future will be "będą odwiedzać/odwiedzały/odwiedzali", you went with one syllable too far :)
Regularly... the best options are probably "we wtorki" (on Tuesdays) or "co wtorek" (every Tuesday). Instrumental for singular doesn't really work I think, Instrumental for plural works in some rare constructions, but that seems too advanced to discuss, I'm not even sure when exactly.
The thing is that what you call Future Tense of perfective verbs is called Present Tense in Serbian grammar (I guess since it's built like Present?). However, this so called Present is used only to explain actions in future.
We should call it Future as well, right? :D