Subject- Verb Agreement

Subject- verb agreement: When a subject doesn't agree with its verb in number.

For example: 

Tonight, Mike and I are hanging out.

The underlined words are the subject and the verb. There is no error in this sentence because 'Mike and I' are plural, and 'are' is plural, thus agreeing in number.

The child sings beautifully. 

The underlined words are the subject and the verb. There is no error in this sentence because 'child' is singular, and 'sings' is singular, too.

School are boring.
The error in this sentence is obvious. School is singular, and are is plural. 

The problem is that there are words that are usually thought to be singular or plural, but they're not. Here's a list of some of these words:

Singular words:

The- Everyone- Everybody- Everything- Some- Nobody- No one- Anyone- Someone- All- Each- Neither (without nor)- Either (without or).

Plural words:

Either/ or- Neither/ nor- And- A.

When you see any of these conjunctions in a sentence with more than one subject, the 1st subject determines the verb:
As well as- together with- along with- in addition to.

When you see any of these conjunctions in a sentence with more than one subject, the subject closest to the verb determines the verb:
Neither/ nor- either/or- not only/ but also- not only, but/ as well.