Based on the definition of prefixes and suffixes, I think "under" is a prefix in the word "underdog" but not in the word "understand". This is because from their meaning, "an underdog" is a predicted looser of a competition while which denotes they're inferior or they're under. In the word understand, the word "under" doesn't denote anything beneath or anything under. I don't think my explanation is as clear as you'll want it to be. I don't want to give you a clear answer without letting you see the rules in which I used to obtain those answers. Similarly, the word "wards" in "afterwards" is a suffix in "afterwards" this is because "afterwards" denotes a point in the future. Hope you get the point? If not let me know so I can use a more formal approach. hope it helps? Best Regards!

'Underdog' looks like a regular compound word: preposition + noun. Both words have meaning on their own. 'Toward' might have begun life as a similar compound. But nowadays '-ward' does not mean anything on its own, so it is a suffix added to a preposition and thereby making a new preposition.

No, none of them contains prefix and suffix respectively. They are complete different words.

correct