say a user searches for "bananas". Will my ad with the keyword "banana" appear or will will I have to bid on "bananas" as well?
YSM associates both singular and plural as the same. AdWords on the other had see's them as seperate keywords, so it's good to place both in your list. Although, unless you are using the exact match type, AdWords may or may not still show "bananas" when you are only bidding on "banana" or vise versa. Either way, it's safer just to bid on both.
Depends on your matching options. If you're bidding on "banana" using broad match your ad will show for the plural version. "Broad Match - This is the default option. If you include general keyword or keyword phrases-such as tennis shoes-in your keyword list, your ads will appear when a user's query contains tennis and shoes, in any order, and possibly along with other terms. Your ads will also automatically show for expanded matches, including plurals and relevant variations. " From here.
I actually tried to search for a plural version of one of my keywords and my ad didn't show. there's only 1 other ad so I know for a fact it's not because of bad placement.
My guess is that's due to budget constraints. If your budget is not set at or above the max recomended and you're opting for smooth delivery your ad will not diplay everytime.