Where did the (e)d go?
Sometimes when you read a sentence the grammar seems a bit off, but you aren't sure why.
For example, what if I write "I am suppose to know these things; I have a PhD"? Does that sentence seem correct to you? Some grammar and spell checkers won't be able to find the error in that sentence, because, after all, there are many times when “suppose” is the correct verb form to use. Just consider the sentences I suppose you want more money? or What do you suppose happened? However, in my sentence above, it should say: "I am supposed to know these things."
So why is "suppose" correct sometimes, and "supposed" correct sometimes?