Cities that have hard winters have no “alternative” and must repair roads in the summer. And when they do, they need to provide motorists with “alternate” routes.
That sentence illustrates the difference between “alternative” and “alternate.”
The two words can sometimes “alternate” places with each other. Knowing the nuance each brings, however, can make a sentence more precise, or less.
As a noun, “alternate” means a substitute, as in the first “alternate” on the jury, someone who will step in if another juror cannot serve. As an adjective, it has two similar but distinct meanings. One is the substitute, the “alternate” juror. The other is something that takes turns: “The club will meet on alternate Fridays,” meaning it meets one Friday, does not meet the next, but meets the Friday after that. “Alternate” is also a verb.
As a noun, “alternative” is an option or a choice, as in “My doctor gave me the alternatives of eating more fish, tofu, or beans instead of red meat.” Some usage experts, apparently leaning on the Latin root of “alter” as meaning “the other of two,” have argued that an “alternative” offers only two choices, but they have been shouted down enough that they can be ignored. “Alternative” is not a verb.
It’s where “alternative” is used as an adjective that the differences with “alternate” come most into play. Some of the “choices” involved in having “alternatives” may indeed be considered substitutes, or “alternates,” but not all of them. “Alternative” story forms in journalism, for example, such as graphics or Q & A forms, could be complements to “traditional” forms or be used in their place. But few people call them “alternate” story forms.
In many stories, though, writers use “alternate” when they mean “alternatives,” in effect confusing substitutes and choices. “The City Council was presented with three alternate ways to deal with the budget crisis,” for example, should have been “three alternative ways.” Even better, “three alternatives.” And therein lies a test to see if you want the adjective “alternative” or “alternate”: If you make the adjective into a noun, which would you use? Few people would say “The City Council was presented with three alternates to deal with the budget crisis.”
As a second test, if you can use the noun “substitute,” you probably want “alternate”; if you can say “choice,” you probably want “alternative.” “The City Council was presented with three substitutes to deal with the budget crisis” makes less sense than “The City Council was presented with three choices to deal with the budget crisis.”
Garner’s Modern American Usage lists the use of “alternate” as an adjective when “alternative” is meant at Stage 3 of the five-stage Language-Change Index, meaning it’s not fatal to use it. But given the alternative—lack of clarity in your writing—you should choose the alternate route.