But more important is the fact that traditional study of Latin starts out with a grammatical framework.... As American students begin Latin, they become acquainted with the "Latin grammar" system, which they can indirectly transfer to their work in English. What it gives them is a standardized set of terms in which to describe words in relation to other words in sentences, and it is this grammatical awareness which makes their English writing good. -- Professor William V. Harris, NYU
The grammar program built into Microsoft Word has chosen to underline the final word in the above quotation. Incorrectly, I should add. Obviously, MS Word has not studied Latin! If it had, it would certainly know the difference between facit bene and facit bonum.
As a teacher of both English and Latin, I have seen the futility of attempting to teach English grammar to students in their native language. For some reason, we resist systematizing that which comes so naturally to us. So we must approach our own language by a different road, and the best road for that is Latin.
Careful attention to Latin grammar drastically reduces, I will not say eliminates, the need to study English grammar as a separate subject, and if English grammar is studied in conjunction with Latin grammar, say in the 1st-4th grades using a program like First Language Lessons by Jessie Wise Bauer, then there is a synergistic effect benefiting both languages. In short, learning Latin grammar improves writing skills in English.