The following may help you understand the statement that "all" can be singular or plural.
Singular use of "all": With non-count nouns, "all" goes with the singular form of the noun. As the subject of a sentence, it also goes with "is", "was", or any other singular form of a verb:
All of the meat was spoiled. All the butter has melted. Not all chocolate is sweet.
Susan and Larry are fond of all antique furniture. Mr. Burnett has provided all of the information we requested. Nobody will be able to eat all this food.
Plural use of "all": With count nouns, "all" goes with the plural form of the noun. As the subject of a sentence, it also goes with "are", "were", or any other plural form of a verb:
All of the children have arrived. All men are mortal. All the doors were open.
Ben likes to play all kinds of sports. We have interviewed all the applicants. The director spoke to all of the employees about the new rules.
The following are all incorrect:
All of the child has arrived. All man is mortal. All the door is open. ... and so on.
For the purposes of the present discussion, the personal pronouns may be considered countable, but with irregular plurals, not unlike "child", "children", perhaps. The singular/plural pairs (subject forms, object forms) are:
I/we, he/they, she/they, you/you; me/us, him/them, her/them, you/you.
As a consequence of their status as countable, "all" goes with only the plural form of these pronouns, the same as for any countable entities.
Correct: "we all", "they all", and "you all" (when it refers to more than one person). Similarly, "all of us", "all of them", ... "All of us / We all like dessert."
But not correct: "I all", "he all", "she all", and "you all" (when it refers to only one person) Similarly, "all of me", "all of him", ... "All of me likes dessert."
Now that said, there are some uses of "all" with the singular of countable entities, usually idioms of an exceptional nature. But I leave that for another post another day.
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