Weak and strong forms of the word "just"

Hello, I'm studying Ponetics and Phonology, and one of the doubts that came to my mind is: When do we pronounce the word "Just" in its weak, and when in its stong form? Also I have problems with another word: "Must". The same question.
I know that there are differences , but I don't have the necessary information I need to give my exam. And I don't find that information in my Phonetic book. Thank you very much for your help!