I know in Korean that the word 온 means "all" that is used to modify nouns (eg 온 가족, 온 인류). I also know that the word 모든 also means "all" and is also used to modify nouns. So What is the difference between 온 and 모든? Can I say "모든 가족" or "모든 인류"?
온 is close to "whole" in English while 모든 is more like "all" or "every".
So 온 loosely refers to the whole group (family, organization, country, etc), stressing not excluding anyone out. It is used for group of people or things that are considered as a close group, implying they belong together.
So for 가족 and 인류, 온 usually works better. Since 모든 is more like "all/every", 모든 가족 might be taken as "all the different families", whereas 온 가족 means the whole of one family. 온 나라 and 모든 나라 have similar issue, but if it's a single entity like 인류, both can be used.
For abstract things or when the individuality is important, 모든 works better. For example, 모든 일에는 순서가 있다, 모든 돌발 상황에 대비해야 한다. These are about separate, unrelated things, so you can't use 온 for them. And even for people, 모든 is appropriate when referring to the individual situations, as in 나라를 위해 죽어간 모든 사람들.
Also, anytime you have a long descriptive clause, 모든 usually sounds better. 온 typically creates a simple noun phrase without another determiner or adjective (e.g. 온 동네, 온 나라, 온 천지, 온 세상). I think we tend to think of these phrases as single words.