Basic Rule.
A singular subject (she, Bill, car) takes a singular verb (is, goes, shines), whereas a plural subject takes a plural verb.
for example:
The list of items is/are on the desk. If you know that list is the subject, then you will choose is for the verb.
An invalid is someone who is sick or unwell.
After we lit the busen burners, we were able to add the chemicals
