Today, I encountered with this sentence "All people know is what you tell them" and I totally got confused why the author uses the verb IS in the sentence. I think "All people know" is plural that's why I'm confused. Could someone help me out ?
all = the only thing
The only thing people know is what you tell them.
Here is another example: All I can do is call John and ask for help.
The subject of the sentence is a Noun Phrase, and it is singular.
A noun clause can be the subject of a sentence.
It can be singular or plural: All the books I've read are purple.
Basically, you can replace "All people know" with "People's knowledge", in which case, knowledge is a singular.
Collective nouns (people, company,and the like) can use either a singular or a plural verb, depending on the meaning of the sentence.If the noun is used to refer to the group as a whole, a singular verb is used; if it refers to the individuals within the group, it takes a plural verb. In the example you gave, it is the group 'people' that is meant, not the individuals that comprise'people.' https://webapps.towson.edu/ows/moduleSVAGR.htm