At first, I wash taught that ещё means "yet". However, as I get deeper into the grammar, I'm seeing that sometimes it means " more" or even "else"! What is the actual meaning, and how can I differentiate between the meanings?
The meaning of the word ещё can't be easily explained. In short, it grants the next word the meaning of delaying in terms of time or in terms of material things. It's like when the instance of something was already mentioned, and the word ещё represents another instance of something from the speaker's perspective. If you're familiar with German, noch carries the same meaning.
It can be translated into English in many ways depending on the context:
Я (всё) ещё не сделал(а) это.
I have not done it yet. / I still haven't done it.
Смотри, какое огромное здание. — О-о, достаточно большое, но я видел здание ещё больше.
See, how huge that building is. — Oh, that one is quite big, but I saw one that's even bigger (more + adjective).
У меня есть эта вещь. — А у меня ещё одна
I've got this thing. — And I have another one/one more
Я хотел(а) бы ещё добавить этого
I would like to add more of this
Also note, the sentences Я хотел(а) бы ещё добавить этого and Я хотел(а) бы добавить ещё этого have different meanings. The last one means in addition to what's already there, I would also like to add this.