To refer to two people/things within the same group, use 'both (of) + (the/my/etc.) + plural noun'.
- My two sisters are rich.
- → Both of my sisters are rich.
- → Both my sisters are rich.
We also use both with other verbs.
- My parents both work in the same company.
and with modal verbs (can, should, would).
- My parents can both speak English fluently.
'All' means 'including every one/part of a group'. We can use 'all' for:
- Countable nouns: an entire group of people/things
- Uncountable nouns: the entirety of something
For (1) countable nouns, 'all' is plural. For (2) uncountable nouns, 'all' is singular.
- All the girls in the class are intelligent. (= 'Girl' is a countable noun, so in this sentence, 'All' is a plural.)
- All the water in the pool is gone. (= 'Water' is an uncountable noun, so in this sentence, 'All' is a singular.)
We use 'each' when we want to focus on the individual members of a group. When you go to a store to buy apples, you may see signs that say '$1 each'.
- Each student has a book. (= all the students in the group have a book)
- Each of the students has a book.
“Each” can also be used as a pronoun as in the second sentence above. In this case, it is followed by a singular verb.
- One room has a TV.
- Each of the rooms has a TV.