Compound words are a spelling problem, not a vocabulary problem; to hyphenate or not to hyphenate is usually the issue. Unfortunately, there's no simple answer. Compound words are sometimes spelled solid (halfback, oversight, torchbearer, halfhearted, midweek), sometimes written as separate words (top hat, decision maker, reddish orange), and sometimes hyphenated (half-moon, self-knowledge, one-half, able-bodied). Also, over time some words lose their hyphens and are spelled solid. Some rules apply, but there are too many to memorize, with too many exceptions. For example, while wagon train and pool table are each written as two words, wagonload and poolroom are each written as one, with no rule to explain the difference. The best thing to do when you aren't sure is to consult an unabridged dictionary. If the compound is a noun, you have a good chance of finding it. If it's an adjective, your chances are fair. If you can't find the word you're looking for, you can try applying some general principles.
Current trend in spelling compound words
The current trend is away from hyphens, as in handshake, notebook, taxpayer, poolroom, crosswalk, etc. When a compound is temporary (that is, used for a particular purpose or not in the dictionary), you can either hyphenate it or spell it as two words: quasi-normal, pool cover.
Compound adjectives
Many, but not all, compound adjectives are hyphenated when they appear before nouns (cross-country trip, full-length mirror, half-baked scheme but midweek meeting, worldwide circulation, halfhearted support). Again, check your dictionary. When a compound adjective is temporary and not in the dictionary, it is customary to hyphenate it: horseshoe-shaped driveway, top-ranked athlete, velvet-trimmed coat. Using a hyphen is especially important if the compound adjective could mislead a reader. For example, fast-moving van means a van that is going fast, whereas fast moving van means a moving van that is going fast.
When a compound adjective follows a noun, the hyphen is omitted: The athlete was top ranked; The driveway was horseshoe shaped; The coat was velvet trimmed; The van was fast moving.
Compound adverbs
While most compound adverbs are written as two words (distributed all over, going full speed), those adverbial compounds beginning with over or under are spelled solid (overeagerly, underhandedly). Adverbial compounds consisting of spelled-out fractions are hyphenated: two-thirds completed.
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