'As pure as the driven snow'

Students often produce funny (sometimes 'haha' funny and sometimes 'strange' funny) English in my classes, but I always appreciate their efforts to use new expressions in their speaking or writing. However, teaching students how to use idioms or expressions in appropriate contexts is quite challenging. It's often not possible to supply simple synonymous phrases, and many ambitious students want to try out their new expressions immediately without attending to the contexts I (or a book) provide for an expression's usage.

For example, recently, a student wrote in his journal that what he liked best about San Diego was "the ocean, as pure as the driven snow.' It struck me as quite funny at the time I read it (I laughed), but explaining how to use this expression was not that easy.

A student has to understand that this 'pure' evokes an image of newly fallen white snow, which hasn't had time to get dirty from particles in the air falling on it or from vehicles driving over it. In addition, we usually use that expression to describe the nature of a person who is innocent, virtuous, or unspoiled. Thus, 'the ocean, as pure as the driven snow' doesn't work on two counts. First, the ocean is usually thought of as a colored, fluid entity, unlike newly driven snow which generally piles up into a powdery white mass. Second, the student's metaphor refers to some thing's appearance rather than to someone's basic character.

What is difficult to decide as a teacher is whether to encourage the misuse of an expression (i.e., complimenting the student for using 'as pure as the driven snow') in order to avoid discouraging the student from experimenting with idioms or phrases that (s)he doesn't yet completely control, or to discourage misuse by immediately correcting the student and re-emphasizing correct usages. I know it's not quite so black and white, but finding the right touch or balance between encouraging use of new vocabulary and correcting misuse is a daily dilemma.