Not English Idiom

OK, the hard part about English grammar is the many verb tenses, right? It is stressful if you ever actually have to think about verb tense. (Was that last sentence in the present, present perfect or present progressive?)
I have found some even more nefarious villains in our language, however. They have been billing themselves as innocent little helpers; so insignificant, in fact, that they are not capitalized in a title. I am talking about prepositions. Their job? Explaining how a noun or pronoun relates to another word.

As we huddled around the woodstove at the schoolhouse back in Gratiot, Teacher used to tell us that a preposition was “anywhere a squirrel could go.” (Sorry—I keep forgetting I’m not Laura Ingalls Wilder.) What would that squirrel think if it knew that the part of speech that made it famous was robbing non-native English speakers of precious points on the SAT? Probably nothing—squirrels are pretty stupid.

I realized prepositions were a problem when I recently taught SAT Writing and Grammar to a class of Korean-American students. I was having trouble explaining why some of their answers were incorrect. About two-thirds of the time, prepositions can be taken literally—just visualize that squirrel on the rail, going through the woods or riding in a Ferrari. The other ones are problematic, however. They just sound right to a native speaker, which means they just have to be memorized by one who is not. The answer key simply says “not English idiom”—how helpful.

English is rife with idiomatic phrases that way-lay ESL speakers. Margarita, a woman in my book club, says that her husband thought she had been calling him a pig for years. Every time she made a new dish, she would ask him to be her guinea pig. Imagine his relief to find out that he was just an experimental subject instead!

Back to prepositions…
Does one work “in” or “at” a factory? Why do we say in the morning, in the afternoon and at night? If one were to say “I have trouble sleeping in the night” an English speaker born in America would notice that it was an unusual phrase or not an English idiom. (Best to take some Ambien and avoid the whole scenario; what’s more American than taking a highly advertised prescription drug?)

OK, so which is right? I work in my father’s factory or I work at my father’s factory? It’s the latter. The word in usually means "within a geographical place or enclosed area”; in this sentence, the preposition refers a location that involves a specific activity, so at is correct. Damn you, squirrel!