Original sentence: "What do you think is Chaucer's understanding of masculinity?"
MS Word suggestion: "What do you think is Chaucer understands of masculinity?"
Certain verbs cannot be paired with forms of the verb "to be." Use the simplest form of these verbs (without the "ing") when you write about present or past action.
Instead of: "Eris was preferring the opera to rock music."
Consider: "Eric preferred the opera to rock music."
Instead of: "Jonathan is needing a break in his studies."
Consider: "Jonathan needs a break in his studies."
How the heck does the computer think that "Chaucer's" = "Chaucer is", when it's immediately preceeded by an "is"!? So the computer thinks it's correcting "Chaucer is understanding ..." to "Chaucer understands". And, weirdly enough, when it is corrected to "What do you think is Chaucer understands of masculinity", the computer doesn't complain. Looks like grammar check still has a loooong way to go before it's considered useful.