Compound words are formed when two or more words are combined to give a new meaning. There are three main ways of writing compound words: as hyphenated compounds (joined by a hyphen, e.g., front-end), as closed compounds (i.e., forming a new single word, e.g., waterproof) or as open compounds (spelled as two separate words, e.g., first aid) Compounds can also be formed by more than two words (e.g., sister-in-law). Thus, a common dilemma writers contend with is deciding whether a compound word should be hyphenated, left as separate or combined as a single word.
Open Compound Words
These words are formed when a noun and its modifying adjective are combined to yield a new noun. However, this doesn't expressly mean that a noun and a modifying adjective are compound words. We use a space between both words, so it's not uncommon to easily forget to identify them as compound words. On the whole, two or more words commonly used together are likely to form compounds, e.g., food chain, football boot, good day, etc.
Adverbs with –ly endings can combine with another word, but the compound word derived is spelled separately. For example, awesomely crafted, critically ill, beautifully designed, etc.
Closed Compound Words
Two words can be joined together to form a single unit compound word. Such words were initially not used together but have overtime being integrated and accepted in the English language. Examples of closed compound words you may be familiar with include: foolproof, scorecard, storekeeper, etc.
Hyphenated Compound Words
Using hyphens in compounds follows a plethora of rules. But an important one to remember is that compound adjectives are only hyphenated when placed before the modified noun but not after it. Here are some examples:
A short-term measure
Those old-time tips don't work in the era of technology.
When used after a noun, we have:
The measure is only good for the short term
This method looks old time.
The examples only illustrate one of many rules guiding the use of hyphens in compound words. If you are not quite sure whether a hyphen is right or not, always look up the word in a dictionary before taking the plunge.