"These olives are not in the market."
Translation:Hae olivae non sunt in foro.
There are no rules per se - Latin has an extremely free word order (the default is SOV, not the rule). It depends on what you want to emphasise.
Also, I think it is common practice to have the object after the verb if there is a preposition (e.g in foro). It has been a while since I studied Latin however, so please excuse me if this is erroneous.
And also, "in foro" has to not be splitted,. No "foro sunt in" for instance.
There are rules for the position of "to be", not really "rules", but the most common word order (SOV) changes with "to be".
Statistics about Latin texts show that "to be" is more common at the beginning of the sentence, or in the middle of the sentence.
Olivas when it is the direct object (accusative); olivæ when it is the subject (nominative).
Declension of oliva (first declension, feminine):
I would think that placing the 'non' in front of 'in foro' (which DL accepts, by the way) would place emphasis on the fact that it is in the market that the olives can't be found. Whereas the more conventional or neutral word order would be to put the negator in front of the verb. Am I correct in this assumption?
Try all LatinTutorial's videos: https://www.youtube.com/watch?v=2fhP_fk2wNQ&t=7s
also check Carpe Lanam's Duolingo course in the forums https://forum.duolingo.com/comment/8053459/Latin-Lessons-Directory
Also this site kinda summarizes all the videos: https://classics.osu.edu/Undergraduate-Studies/Latin-Program/Grammar/Cases/latin-case