1. The professor will not accept any late papers
- it is not except, because except means to "not including"
2. We just passed the last gas station for 40 miles
- It is not past, because even though it is a past particle, it is still continuing
3. Can you please sit down and be quiet for 5 minutes?
- It is not set, because sit means to be in a position in which one's feet and one's back is upright, while your lower body (until it gets to the legs) supports the rest of the body.
4. The painting complements the living room
- It is not Compliments, because complements means "to praise"
5. We are trying to determine who's scarf this is
-who's makes it possessive, and gives ownership
hope this helps