For grammar, rightly learned, institutes in the mind a certain knowledge, or process of thought, concerning the sorts, properties, and relations, of all the words which can be presented in any intelligible sentence; and, with the initiated, a perception of the construction will always instantly follow or accompany a discovery of the sense: and instantly, too, should there be a perception of the error, if any of the words are misspelled, misjoined, misapplied,--or are, in any way, unfaithful to the sense intended.
Thus it is the great end of grammar, to secure the power of apt expression, by causing the principles on which language is constructed, if not to be constantly present to the mind, at least to pass through it more rapidly than either pen or voice can utter words.

And where this power resides, there cannot but be a proportionate degree of critical skill, or of ability to judge of the language of others.

Present what you will, grammar directs the mind immediately to a consideration of the sense; and, if properly taught, always creates a discriminating taste which is not less offended by specious absurdities, than by the common blunders of clownishness.

Every one who has any pretensions to this art, knows that, to _parse_ a sentence, is but to resolve it according to one's understanding of its import; and it is equally clear, that the power to _correct_ an erroneous passage, usually demands or implies a knowledge of the author's thought.
But, if parsing and correcting are of so great practical importance as our first mention of them suggests, it may be well to be more explicit here concerning them.

The pupil who cannot perform these exercises both accurately and fluently, is not truly prepared to perform them at all, and has no right to expect from any body a patient hearing.