In Ancient Greek, the position of the accent was limited:
- an acute accent could be on the last syllable (ultima); on the next-to-last syllable (penult); or (only if the last syllable was short) on the third from the end (antepenult)
- a circumflex accent could only be on a long vowel or a diphthong, and could be on the last syllable or on the next-to-last syllable
Vowel length disappeared in the millennia since then, but it still "echoes" in some of the accentuation rules.
-σεις, with a (historical) diphthong, is a long syllable, and so πρότασεις was an impossible accentuation -- the accent moved towards the end to the next allowed syllable: προτάσεις.
η was also a long vowel, so πρόταση would have been illegal in Ancient Greek -- but in Ancient Greek, the nominative was πρότασις with a short iota in the last syllable. The ending was changed to -η in Modern Greek to align this noun with feminine nouns in -η.
Similar accent shifts occur with words such as το μάθημα, plural nominative τα μαθήματα (since τα μάθηματα would have the accent too far from the end), plural genitive των μαθημάτων (since ω is a long vowel and so μαθήματων would again have the accent too far from the end).
But yes -- all feminine words in -η that have plurals in -εις (nearly all of which end in -si, i.e. in -ση -ξη -ψη, the most notable exceptions being πόλη and δύναμη) will shift the accent to the penult in the plural if they are accented on the antepenult in the singular. (So πόλη stays πόλεις but δύναμη becomes δυνάμεις.)
That said: the genitive singular of such nouns is usually της πρότασης, της δύναμης etc. ("impossible" forms), regularised, rather than the ancient forms της προτάσεως, της δυνάμεως etc.
Which look impossible as well, since -ως is long, but the "original original" form was της προτάσηος etc. (with the same -ος of, say, του μαθήματος), but there was quantity reversal where -ηο- ēo turned to -εω- eō. Which is only of historical interest.
It's a bit of a mess that modern Greek tidied up a little but still has to live with.
Thanks again jaye for your help. My french is confusing me again :-p I investigated further and found exactly the explanation which corroborates yours : http://www.wordreference.com/gren/%CF%86%CF%81%CE%AC%CF%83%CE%B7 and https://en.wikipedia.org/wiki/Sentence_clause_structure :-)
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