397. Words are formed from other words in two ways:
1. by adding a suffix to an existing stem, or by slightly changing an ending so as to make a new one (derivation; the new word is derived from the old);
2. by putting two words or stems together into one (composition; the new word is a compound of the two).
a. Declension and conjugation are special forms of derivation, so clearly marked in character that they properly receive separate names. The formation of adverbs has been treated in connection with declension (§§ 228 – 237), but might have been treated here. So with the numeral adjectives, adverbs, and nouns described in §§ 191 and 192.
398. Many Greek stems took their existing form so early that they can not now be analyzed into their parts. For us these are root words, of the original stock of the language. Often their likeness to other Greek words, or to words in kindred languages, throws light on their earlier history; but the study of such relations belongs to comparative philology. In the following sections are treated only the most common types of word-formation; but these alone are enough to show how the bulk of the copious Greek vocabulary has been made, and to assist greatly in learning that vocabulary.