When I asked why not the classes has started, one of my friend suggested a correction: "have started"
My question is: which one is correct?
"Classes" is a plural noun. The verb must also take a plural form to agree with the noun.
One class has started.
Two classes have started.
The present simple of the verb have goes like this:
You will notice that the only case of has is third person singular- he/she/it has.
classes is plural, so the correct verb case is have.