Hyphen when used adjectivally: a half-eaten sandwich; a half-cut subeditor; half-time oranges.

If it's used adjectivally, an end '-e' is used like that of adjectives:

Used adjectivally, tupenny, tupenny-hay'penny, and threpenny were common.

Use the hyphen when extra used adjectivally means that something is unusual because of size or degree (extra-long extra-dry).

Lower case for government when used adjectivally (a government spokesman, government policy); also for state, press.

In everyday language, 'professional' is often used adjectivally of any performance to denote features of that performance which have nothing to do directly with exchange value.

The Daily Telegraph takes the article except when used adjectivally and is italicised.

However, when the noun is used adjectivally it can mean 'each' or 'every': Ilka lassie ha' here laddi.

This behavior is known as "nymphal phoresy" (used adjectivally as "phoretic").