Let us begin, then, with Grammar. This, in practice, means the grammar of some language in particular; and it must be an inflected language. The grammatical structure of an uninflected language is far too analytical to be tackled by any one without previous practice in Dialectic. Moreover, the inflected languages interpret the uninflected, whereas the uninflected are of little use in interpreting the inflected. I will say at once, quite firmly, that the best grounding for education is the Latin grammar. I say this, not because Latin is traditional and mediaeval, but simply because even a rudimentary knowledge of Latin cuts down the labor and pains of learning almost any other subject by at least fifty percent. It is the key to the vocabulary and structure of all the Teutonic languages, as well as to the technical vocabulary of all the sciences and to the literature of the entire Mediterranean civilization, together with all its historical documents.
Those whose pedantic preference for a living language persuades them to deprive their pupils of all these advantages might substitute Russian, whose grammar is still more primitive. Russian is, of course, helpful with the other Slav dialects. There is something also to be said for Classical Greek. But my own choice is Latin. Having thus pleased the Classicists among you, I will proceed to horrify them by adding that I do not think it either wise or necessary to cramp the ordinary pupil upon the Procrustean bed of the Augustan Age, with its highly elaborate and artificial verse forms and oratory. Post-classical and mediaeval Latin, which was a living language right down to the end of the Renaissance, is easier and in some ways livelier; a study of it helps to dispel the widespread notion that learning and literature came to a full stop when Christ was born and only woke up again at the Dissolution of the Monasteries.
Latin should be begun as early as possible--at a time when inflected speech seems no more astonishing than any other phenomenon in an astonishing world; and when the chanting of "Amo, amas, amat" is as ritually agreeable to the feelings as the chanting of "eeny, meeny, miney, moe."
Hurrah! We have actually started doing Latin in our home school. We are approaching it with a very beginner-oriented text, William Linney's Getting Started with Latin. It's made up of tiny, sequential lessons, but after just three weeks of occasional Latin study, we can now say sum (I am), es (you are), est (he/she is), et (and), non (not), nauta (sailor), agricola (farmer), and poeta (poet). We throw in other words that we think are Latin (or not necessarily Latin) when we want to say something outside of what those few words allow. It's great fun. For example, thanks to the film O Brother, Where Art Thou, we can all say to Daddy, "Es pater familias." This morning, we were telling each other we were dinosaurs like this: "Es dee-no-sore-us." I'm sure there's a good Latin word for dinosaur; in fact, isn't the word dinosaur from the Latin for "terrible lizard" or something? Time to go Google.
Update: Silly me. Dinosaur comes from the Greek words for terrible ("deino") lizard ("sauros"). Dead Latin doesn't have a term for dinosaur, but apparently we can use the word "dinosaurum" if we must, according to Google Translate.