What is the singular form of them?

The singular form of "them" depends on the context. It could be:

* he

* she

* it

For example:

* "They are going to the store." -> "He is going to the store."

* "They are very kind." -> "She is very kind."

* "They are a beautiful flower." -> "It is a beautiful flower."

You need more information to determine the correct singular form.