Why do you use hyphen in some words?
Some multiple-word terms are easily read as one word, while
other sets of words need to be hyphenated either for ease of visual
interpretation and pronunciation, or simply to keep the end result
from becoming unwieldy.
Many words we routinely use as a single word today were once
hyphenated; others that today are hyphenated will probably remain
so. For example, the term milk shake can be equally correctly
written 'milk shake', 'milk-shake' or 'milkshake': none of the
three appears especially wrong, though the two-word version is less
common today. One day the term will probably lose its space, and
later its hyphen, forever. The term two-word, though, would look
clumsy written as twoword, and would be difficult to pronounce,
especially for those learning English. But if we write, '… the two
word version …' it could lead to misinterpretation. The term
'two-word', in this case, needs hyphenating, unless we want to
write, '… the "two word" version …', which some might consider
clumsy.
On the other hand, the word 'coordination' is today routinely
written without a hyphen, but some people still prefer
'co-ordination', feeling uncomfortable pronouncing (and we all
mentally pronounce words as we read them) the unhyphenated 'coor-'
part of the word.
It's difficult to pin down hard-and-fast (or hard and fast, but
not hardandfast) rules in these situations, and there seems little
point in attempting to do so, since the only useful aim might be to
settle an argument over which expression is ultimately and solely
gramatically correct in English. This is fine for purely academic
intentions or perhaps to achieve a definitive ruling in a word game
(or word-game, but possibly not yet a wordgame), but for all
practical purposes such questions simply come down to common
usage.
If the majority of people use words or phrases in a certain way,
this is how most speakers of the language will feel comfortable in
expressing those terms; this is, in other words, common usage - the
ultimate decider of right, wrong, maybe, or maybe not, in any
discussion on language. It is the users of any language who
ultimately decide how that language is used.
So in the end you'll very likely be safe to consider how you
feel about the way any idea is expressed: if it sounds good, looks
fine, and you feel okay with it, you're probably right.
