Tip: See my list of the 
MostA common mistake among non-native speakers is inappropriately using the word “of” after “all”. However, it would be wrong to say that this can be fixed by simply omitting “of” every time because there are situations when it is required.
When talking about “all things of a certain kind”, only “all things”, not “all of things” is possible, e.g.
Similarly, when we talk about the total amount of a substance, such as water, we don’t use “of”:
Note, however, that the word “humanity” seems to break this rule, and both “all humanity” and “all of humanity” are in use:
“All of” before pronouns
The situation is reversed when the thing in question is replaced by a pronoun; then the only correct variant is the one with “of” (but sentences employing the pattern “all of + pronoun” can almost always be rephrased in such a way that the pronoun does not follow “all”):
If you want to understand the real grammatical reason why this happens, here’s an explanation: In the examples at the beginning of this article, “all” was a determiner; it had the same function as words like “the”, “my”, “some”, and similar. On the other hand, in the sentence “all of them are animals”, “all” is a pronoun used as the subject of the sentence (“all are animals” is still a syntactically correct sentence) and “of them” only specifies it.
This still does not explain why “all they are animals” is not correct. “All”, as a determiner, is very unusual in that it can be combined with pronouns, but when this happens, the pronoun always precedes the word “all” (and the verb is usually placed between the pronoun and “all”). This is probably due to the fact that allowing “all” in front of a pronoun would often lead to ambiguous sentences; for instance, “all they know” could be understood either as “all that they know” or “they all know”.
All (of) the things
Finally, the case that usually causes the most doubt arises when “all” is combined with “the”, “my”, “this”, and other determiners. “All” often functions as what grammarians call a predeterminer. A predeterminer is a word that further specifies a phrase that already contains a determiner.
For example, when you say “my friends are eating pizza”, “my” is a determiner that makes it clear that you speak only about friends of yours, not about friends in general, but it still leaves a lot of room for interpretation—are all your friends eating pizza? Are only some of them eating pizza?
“All” can be used as a predeterminer to further specify a noun. You can say, “All my friends are eating pizza.” However, as we have already seen, “all” can also be a pronoun, and just like we can say “all of them”, we can also say “all of my friends”. Both variants are grammatically correct:
Having said that, the variant without “of” is significantly more common than the variant with “of”, to such a degree that the variant with “of” might be considered unnatural (or colloquial) by some native speakers in certain contexts. My advice is: If in doubt, do not use “of” between “all” and another determiner.
It is also worth noting that only a small number of English determiners can be used as predeterminers, namely all, what (as in “What a mess!”), a lot of (“a lot of my friends”), lots of (“lots of the people”), and both (“both the parents are …”). Other determiners are never used as predeterminers. We cannot say “many my friends”, “those the books”, etc., so we are forced to use “of”, as in “many of my friends” and “those of the books”.
