I still do a certain amount of work for them.

我仍旧为他们做一定数量的工作。

You have to allow for a certain amount of error.

你必须考虑到一定数量的错误。

People who earn under a certain amount are not liable to pay tax.

收入低于一定数额者不必纳税。

You are going to have to take a certain amount of criticism, but you must cope with it.

你会受到一定程度的批评，但这是你必须面对的。

A certain amount of flexibility is built into the system.

该体系加入了一定的灵活性。