Grammar is the art of speaking and writing correctly. It concerns significant speech, that is, speech that signifies something, and the arrangement of parts as words and phrases and clauses, so that what we mean can actually be communicated.
What grammar studies
Grammar treats the word as a conventional sign and asks how such signs are put together in a given language. A Semitic language like Hebrew where most words are formed from a three-letter root will take its conventional shape differently from a language like English that has a great deal of variety in word formation. Grammar studies letters and sounds, the parts of speech (noun, verb, adjective, preposition, and so on), and how words are joined into sentences.
Grammar and logic
A sentence can be grammatically correct and false, and it can be incorrect and true. Walshe gives a pair of examples: the grammarian judges “man is stone” correct and “man are animal” incorrect, while the logician judges the first false and the second true, though badly expressed (Commentary on the Categories, “What is Logic About?”). So grammar asks whether something is correctly said; logic asks whether what is said is true.
For the same reason, the parts of speech are not the same as the categories. “Man” and “humanity” differ in grammatical form, but both fall under the logical category of substance. “Blue triangle” and “equilateral triangle” have the same grammatical form, adjective and noun, but the first joins something that only happens to be in its subject (i.e., triangles only happen to be blue) and the second joins two aspects of what makes a thing to be what it is, its essence. Grammar and logic consider the same words in different ways.
Why grammar comes first
Logic works on statements, and a statement is made of words. Rhetoric directs a speech to an intended audience, but there is nothing to direct if the speaker has not said or communicated anything. So, the classical tradition begins with grammar: the words must be correctly joined before anything can follow logically from them or be communicated to an audience.
Continue to Studying Grammar.