There's no formula for knowing whether something is one word or two. Often, it depends on whether it's a noun or a verb. Other times, it depends only on which dictionary you're looking at.
But there are a few that you may want to commit to memory. They are "every day" vs. "everyday" and "line up" vs. "lineup."
"Everyday" gets misused a lot, especially by businesses that make claims like, "Great values everyday" or "We're happy to serve you everyday." In both those examples, "everyday" is wrong. Webster's New World says that the one-word version is an adjective and adjectives, as you know, modify nouns. So while you can say, "We offer everyday values," you need the two-word version if it nolonger modifiesa noun: "We offer values every day."
"Lineup" gets written wrong a lot. As a noun, it's just one word. "We added two players to the lineup." But as a verb, it's two words. "We will line up the players on the field." A lot of people hyphenate the noun, "line-up." But the dictionary says not to.
Hyphenating nouns and verbs is different from hyphenating modifiers, that is adjectives. The hyphen's biggest job is to connect two or more words to form a single adjective: "a fast-moving train," "a money-management seminar."
That's different from nouns and verbs, whose hyphens are determined not by any formula, but only by whether your dictionary includes a hyphen in their official spellings. The most striking example: the noun "water ski" takes no hyphen, but the verb "water-ski" does. The only way to know that is to look it up.
Dictionaries disagree on a lot of these matters and their disagreements can cause a lot of confusion. For example, you could be reading a news site one day and notice that the noun "healthcare" is one word, then an hour later you see it in a book as two words. That's because "Webster's New World Dictionary," which is followed by many news organizations, says it's one word, but "Merriam-Webster," which is followed by many book publishers, says it's two.
For "childcare vs. child care, cellphone vs. cell phone and makeup vs. make up, the best thing is to just look them up. Then, when you forget them, look them up again.
As for soymilk, there's no need. I really did look that one up and, in "Webster's New World," it's one word. I wonder how long it will take me to forget that.
JUNE CASAGRANDE is author of "It Was the Best of Sentences, It Was the Worst of Sentences." She can be reached at JuneTCN@aol.com.