In particular, grammar is the study of groups or classes of forms as regards their occurrence in frames or slots constituted by other classes. All forms which behave alike in this respect are members of a form class. For example,吃了饭 chyle farm ‘have eaten (one’s) meal’, 打过球 daa. guo chyou ‘have played ball’, and 骑着马 chyij maa ‘riding a horse’ are all of the class of verbal expressions, and within each of them, chy, daa,and chyi are all of the class of transitive verbs, -le, - .guo and -j are all of the class of verbal suffixes, and fann, chyou, and maa are all of the class of nouns, which, among other things, can fill the frame or slot of objects after verbs. Note that, since a form is a class of tokens, a form class is a class of classes.
 This characterization is sufficient for our present purposes. On closer analysis of the situation, especially when modern means of communication and recording are considered, the relations are more complicated, on which see my study of “The Logical Structure of Chinese Words”, Language, 22: 5-8 of 4-13(1946).
 Chao Y R. A grammar of spoken Chinese[M]. Univ of California Press, 1965.