Native speakers rarely, if ever, use simple prepositional phrases incorrectly, and when they do, the problem is usually one of reference. One student, for example, wrote "At the age of thirteen, my father obtained custody of me." To help students avoid such errors, teachers should NOT focus on them. As students place parentheses around prepositional phrases and draw arrows to the word each modifies, errors such as this one will become apparent to the students themselves, especially in view of the pyscholinguistic model of how the brain processes language.

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     Prepositional phrases do often contribute to errors in combination with other constructions. The most widely recognized of these is the slipped pattern^* in which the object of a preposition is confused with the subject, thereby resulting in a subject/verb agreement error, as in "Neither of these are very difficult."
 

"In writing letters to friends or relatives, you not only need to know the standard usage of words but it is also important, to whom you are writing to, that you keep them interested."

* Mina Shaughnessy's term in Errors & Expectations