Recap.  What are Indefinite Pronouns ?

• An indefinite pronoun refers to an non-specific pronoun in a sentence.

E.g.  everyone, someone, each, one, nobody, anyone, some, most, all etc.

• When we say “Anyone can learn to swim”, we are not saying who specifically.

1.  Special Singular / Plural Indefinite Pronouns

• Indefinite pronouns such as all, none, some, more, most and any when followed by a phrase, the verb agrees with the object of the phrase.

E.g.

Most of the boxes have been collected.

None of the muffins are delicious.

• Take note that if the noun is an uncountable noun, then the verb remains singular (sources: almost every international grammar website).
• However, it has been brought up by teachers that schools here only accept "is". Always check with your teachers if there is a doubt.

E.g.

Some of the water is frozen.

All of the sugar is eaten.