1. Some Aspects of Psychological Predicates in English;Aijmer,1972
2. I think — an English modal particle;Aijmer,1997
3. On the grammaticalization of the parenthetical I'm afraid;Akimoto,2002
4. Truth conditional discourse semantics for parentheticals;Asher;J. Semant.,2000
5. Longman Grammar of Spoken and Written English;Biber,1999