We have many words that have grown from the merging of two words. Some have changed over time and merged into a single word.
keyboard, classroom, childlike, redhead, drawbridge, football, paperwork, stepfather
Some require a hyphen.
washing-up, daughter-in-law, e-mail, master-butcher, six-pack, school-run, head-lamp ...
Others seem to work well as separate words:
post office, job centre, vice president, middle classes, fuel tank, car park ...
There is no foolproof way of knowing whether a compound is a single word, needs a hyphen or will stand as two separate words. The computer grammar checker will help and so will a good dictionary (many available on-line). In some instances, there seems to be no universally accepted of writing the word.
For example, semicolon and semi-colon both seem to be acceptable.