Words can be combined to form compound nouns. These are very common, and new combinations are invented almost daily. They normally have two parts. The second partidentifies the object or person in question (man, friend, tank, table, room). The first parttells us what kind of object or person it is, or what its purpose is (police, boy, water, dining, bed):
What type / what purpose | What or who |
police | man |
boy | friend |
water | tank |
dining | table |
bed | room |
The two parts may be written in a number of ways :
1. as one word.
Example: policeman, bestfriend
2. as two words joined with a hyphen.
Example: dining-table
3. as two separate words.
Example: fish tank.
There are no clear rules about this - so write the common compounds that you know well as one word, and the others as two words.