Word for the Wise June 22, 2007 Broadcast Topic: Troop
More than one person has asked for the story behind the word troop. In short, the question comes down to this: if the plural troops is synonymous with the plural soldiers, is the singular troop the same as the singular soldier? (来源：英语杂志 http://www.EnglishCN.com)
The answer is: not really, although you couldn't tell that by the headlines, which commonly feature such phrasing as "Three Troops Killed."
Troop is a linguistic oddity; it sometimes functions as a count noun and sometimes as a non-count noun. Like snow and money, troop is a non-count noun; it is often used attributively (like an adjective) before another noun (think "troop level," "troop count," or "troop movement," for example). When used in this non-count sense, troop refers to a collection of people, usually a military unit.
But of course, troops also acts as a count noun plural, meaning "soldiers," as in "three troops." What makes this even more complicated is that troop can be pluralized to mean "a number of units of soldiers," as when we hear "The army sent three troops of MPs to the border."
But troop is not generally considered a singular count noun and using it that way is not considered standard (except, perhaps, in headlines).