Hi Jian
Sokoon is when there is a pause over a word or a letter is not followed by a vowel. In case you know any Farsi, in the word “Ketab” the letter b has “sokoon“ because it is not followed by a vowel.
Hamze is a strong vowel that has different uses... it is mostly when There are two vowels in a row and they become stronger. If you have a particular exam in mind I’d be happy to help you with how it actually works.