Greeting someone:

The best and simplest way to say hello to somebody in Chinese is

This can be used any time of the day, for greeting any person, be it young, old, man woman, friend, collegue, or a stranger. It should be noted though that close friends normally do not say this to each other, because it sounds a little too formal in this situation. They just nod their head or smile at each other, or say 嗨 (hāi), which, as you guessed, is derived from English.

In the early morning, you can also say

Or this in the evening: