1. Some Proposals Concerning the Modal Verb in English;Anderson,1971
2. Das semantische System der deutschen Modalverba;Bech;TCLC,1949
3. The Semantics of the Modal Auxiliaries in Contemporary German;Bouma,1973
4. On Contrasting the Semantics of the Modal Auxiliaries of English and German;Bouma;Lingua,1975
5. Time, Tense, and the Verb;Bull,1963