When should I use hyphens?

It's complicated. There are some rules but they are loose and they vary in different countries.

Here are three rules to help you along:

- hyphenate compound modifiers before nouns

- hyphenate between a word and a prefix if you want and if it looks better

- don't hyphenate two nouns that "belong together"

Hyphenate between a word and a prefix if you want and if it looks better

Never use a space for prefixes. You can hyphenate prefixes all the time if you want to. Some words are hard to read without hyphens, particularly when there are two vowels next to each other. In these cases you should always use a prefix.

Examples:

pre-historic or prehistoric

pre-diabetes or prediabetes

pre-existence (preexistance looks strange)

pre-industrial (preindustrial looks strange)

Don't hyphenate two nouns that belong together

In English, two words that belong together are not usually separated with a hyphen, they are separated with a space.

Examples: 

a tablet computer

the hospital food

a mobile phone or a cell phone
the biogas process

If the word really takes on a new meaning, it is often joined together as one word.

Examples:

smartphone or smart phone



Origin of the confusion:

In German, it used to be easy. Two nouns that belonged together would be stuck together. Donaudampfschiff. Now things are changing, and people are starting to use hyphens instead of putting words together, to give Donau-Dampf-Schiff. People think this comes from English, but you don't do this in English! It would be a Danube steamboat (a Danube-steam boat would be a boat that uses steam from the Danube, and a Danube steam-boat is just wrong).


P.S. There are always exceptions

Hyphenate compound modifiers before nouns

In British English, all compound adjectives should be hyphenated.

Example: 

a well-written article

nitrogen-fixing bacteria

In American English, it's not as important unless there is a loss of meaning without a hyphen.

Example:

a small-appliance company (company that sells small appliances)

a small appliance company (small company that sells appliances)

If it's not before a noun, it usually isn't hyphenated.

Example:

The advanced-physics teacher teaches advanced physics.

The text was very well written.

This website is quite useful.