Please explain when we should use א or not. It seems to be added to some words but not others that have exactly the same vowel. For example, why do "hoo" and "hee" need an א at the end, instead of just הו and הי? Duolingo does not provide the explanation for this. It seems random.
Yes, ש can be pronounced both as "s" and "sh". When nikkud is used, the difference is visible by adding a dot above the letter:
when the little dot above ש is on the left side, it's pronounced as "s". - שָׂמֵחַ "sameach"
when the little dot is on the right side, it's pronounced as "sh" - שׁוֹתֶה - "shote"
After some practice I figured it out! They are all vibrating sounds made in different parts of your throat: Resh is like gurgling; het is deep, like when you choke and kaf is in between, more shallow than het (try saying "kh", like trying to get rid of something that stick to the deepest part of your palate. Imagine you are trying to speak in a dry and dusty air of the desert ;-)