![Tongue Tied [:S]](/emoticons/emotion-7.gif)
The article you quote is about the relationship between
car and
its.
Every
car must have
its brakes tested.
The article is not about subject-verb agreement. But since you ask, there are two grammatical facts to consider:
1. The modals (
can, will, must, ...) do not change form in the third person present.
2. Only the base form of a verb can be used after a modal.
have is not a modal verb.
has is singular;
have is plural (in the third person, present tense):
Every car has wheels. The car has wheels.
All cars have wheels. The cars have wheels.
must is a modal verb.
must is both singular and plural:
Every car must be counted. The car must be counted.
All cars must be counted. The cars must be counted.
must is a modal verb. If
have is used after
must, it can only be in the form
have (the base form), never
has.
Every car must have wheels. The car must have wheels.
All cars must have wheels. The cars must have wheels.
Therefore,
Every car must have its brakes tested. (
must has is wrong!)
Every car has been tested. (
has is singular,
has is not placed after
must.)
CJ