Vivian Cook  blends  infixes plurals  words site 

A compound is formed when two words are added together to get a new word. Sometimes the meaning may just add the two words together; a madman is a man who is mad.  But usually the meaning of the compound is more than the sum of its parts. In British usage you might guess that a girlfriend is a friend who is a girl, that a sheepdog is a dog that looks like a sheep or that an electric chair is a wheelchair with a motor.

However, the links between the two words in compounds are as complicated as the grammar of the whole sentence. Here are some of the possibilities:

- madman (a man who is mad), blackberry (a particular type of black berry), highchair (a particular kind of chair that is high): the first word is an adjective that adds to the meaning of the noun

- heartbreak (the state of a heart that breaks), snakebite (when a snake has bitten), cloudburst (a storm like a cloud bursting): the first word is a subject that goes with the following verb

- watchmaker (a person who makes watches; bookseller (a person who sells books): the first word acts as the object of the following verb with ‘-er’ added

- fishing rod (a rod for fishing), ironing board (board for ironing), carving knife (a knife that carves): the first word carries out an action of the second.

But there are a large number of other ways of making compounds, illustrated in compounds with dog:
- a lapdog (a dog that sits on a lap, with metaphorical uses),
- a puppydog (a dog which is a puppy)
- a bulldog (a dog which is like a bull)
- a sheepdog (a dog that herds sheep)
- a police dog (a dog used by the police)
- a watch-dog (a dog that watches, with metaphorical uses)
- a hunting dog (a dog used for hunting)
etc.

Often there are possible pairs of words, one a compound, one a simple combination. Take colours:

    a white board/a whiteboard
   the white house/the White House
   a black bird/a blackbird
   a black berry/a blackberry
   a blue bottle/a bluebottle
   a red cap/a redcap
      a blue tooth/Bluetooth
   a black bottom/the blackbottom

This also demonstrates the lack of consistency over how a compound is spelt. Sometimes there is a space between the words, The White House, sometimes a hyphen tea-time, sometimes neither, blackberry. The longer the word has been in English the more likely it is to have lost its space and its hyphen. The OED records a progression from tea bag 1898 to tea-bag in 1936 and teabag in 1977. Pronunciation may be a better guide: compounds tend to be stressed on the first word the White House, non-compounds to have even stress the white house. A famous example debated by linguists is the lighthouse versus the light house, or even better the lighthouse keeper versus the light house-keeper.