You must (be/have/do something)

Use this expression when you want to make a guess about someone. It's really useful when you're trying to be nice to someone and predict what they want. For example, you might say this to someone who's a guest at your house:

You must be starving!

This phrase also shows that you understand a person's feelings:

You must have been disappointed when you found out you didn't get in.

quite a (something)

"Quite" is an adverb that means something similar to "very". It's perhaps a little lower in strength than "very".

If you're using "quite" with an adjective, you say "quite ___":

I'm quite hungry.

If you're using it with a noun, you say "quite a ___":

It's quite a useful book.

You have quite an impressive background.

It's also an intelligent or sophisticated-sounding word, so some people who want to seem intelligent use it more while people who want to seem more "down to earth" use other words like "really ___" or "real":

You must really have a sense of accomplishment.

feel a sense of accomplishment

To "feel a sense of accomplishment" means to feel like you've achieved something great. Here are some events that might make you feel a sense of accomplishment:

winning a sports tournament

getting a really good job

doing your taxes two months early

In order to "have a sense of accomplishment", you need to do some work first. If you won a lottery, you would feel great, but you probably wouldn't feel a sense of accomplishment.