When a subject in a sentence is in the singular, then the verb must be in the singular too. When the subject is plural, then the verb is in the plural, in agreement with it. This is also called concord. Examples are:
Paul is at university and so is his brother.
Paul is at university and so are his brother and sister.
They understand the reason why they have to do this.
She understands the reasons why she has to do this and why
you have to do it too.
These conditions apply now.
This condition applies now.
Non-NE writers can forget to check concord in their writing. Two quite typical examples are:
Sara has received our e-mail. Has you received it too? Correct
version: Sara has received our e-mail. Have you received it too?
This kind of topics. Correct version: These kinds of topics.
As a rule of thumb, all you have to do is work out who is doing the action and make your verb relate to who or what is doing it. In some sentences you may have to refer back to check.
Incidentally, there are certain words in English where it is possible to use a singular word in a plural sense too. Examples are: government, council, committee, company.
So in UK English, you can write:
The government is changing the law on this.
The government are changing the law on this.
The reasoning behind this is that these nouns can be viewed as entities by themselves or as bodies of people. On this track, another often-used word comes to mind. This is the word ‘staff’, where it means personnel. It is used as a singular in US English but exists only in the plural in UK English.
So UK English says: ‘The staff are taking a vote on this.’
US English says: ‘The staff is taking a vote on this.’