It’s easy to become confused over the proper use of the humble hyphen. The main purpose of which is to join two (or more) words together, thereby making them a single compound word with its own meaning. As in:

• an ex-President is a former President.
• a co-director works with another director.

The absence of a hyphen can also lead to misunderstanding:

I must re-cover the sofa (with new material).
I must recover the sofa (from the person I lent it to).

After his time in prison, he was a reformed character (no longer a criminal).
They re-formed the band and played in the garage (started up again).

Prefixes like co- and pre- should have a hyphen when next to a word beginning with the same vowel, as in: co-ordinate, pre-empt.

Hyphens can contribute considerably to clarity, as in:

You must read two hundred odd pages a day which gives the impression you are only to read the odd pages, hence the hyphen in: You must read two hundred-odd pages a day.

There are several word combinations which are hyphenated when they come before a noun, but not when they come after, as in these examples.

He is a well-known author.
This author is well known.

She is a part-time worker.
She works part time.

It’s all as clear as mud, I hear you say. Ah, don’t you just love the English language.