When the compound is a modifier, especially when it modifies a noun, we use a hyphen:
This is a well-written article.

But there is no hyphen in the following sentence:
I think the article is well written.

Here are some more examples:

They are five-year-old children. (Notice that there is no plural -s).
The children are five years old.

He is a part-time mechanic.
He works part time.

This is a long-term project.
We must plan for the long term.

The hyphen often helps to clarify what the writer wants to say.

He is selling his little-used bike means that the bike has not been used much.
He is selling his little used bike would mean that the bike is little (small) and has been used.

Do you understand the difference between a single family home and a single-family home?
And between a small business owner and a small-business owner?
Or an old furniture salesman and an old-furniture salesman?

We can leave out the hyphen if there is no risk for confusion:

I found a used car dealer.

An adverb ending in -ly is not followed by a hyphen:

A smartly dressed woman

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