Sorry to contradict, Eliot... but you can never say "The Middle Ages were a time..." 1) both subject and verb are in plural, thus you cannot use the singular indefinite article. And 2) as far as I know, there is ONE period in history called "Middle Age" and since Le Yunyi has written "Middle Age" in capital letters, I am assuming she refers to that period of time and not to "ages within that period" (in which case could be possible, but not using capital letters). And... why "rivalries" cannot be feudal?

1.The Middle Ages was a time of feudal rivalries. This looks fine to me. You could also say: The Middle Ages were a time of feudal rivalries. But you can also use the singular. Or maybe it's because rivalries cannot be feudal. I am not an expert on this stuff so it's just a suggestion. 2.We will have to finish a 12-pages assignment in a week. -------------------------------12-page---------------------------- There is no 's' on 'page'.

3) let me add the 3rd one "a 12-pages assignment" is wrong I think .It must be" a 12-page assignment"Because you refer to "the whole"of the assingment (only one assingment out of 12 pages) and not the number of the pages of it.

1) The Middle Age was a time..... There was only one Middle Age, therefore no 's'. 2) ....in a week Unless you made a spelling mistake, I'm sure it's the last word; 'week', not 'weel'.