Are there any rules to help one know when to pronounce the letter "e" open and when to pronounce it closed? For instance, could I have known, when seeing the word agression for the first time, that it was to be pronounced /agʀesjɔ̃/, and not /agʀɛsjɔ̃/?
Furthermore, Le Petit Robert transcribes the pronunciation of the word terrorisme with a closed "e" – /teʀɔʀism/; the Oxford-Hachette French Dictionary does so with an open "e" – /tɛʀɔʀism/. What could be the reason for this dissonance?