i.e. [id est] that is e.g. [exempli gratia] for example Both abbreviations are almost always separated from text by commas. "The best interpretation, i.e., my opinion, is that ..."
et al. [et alia] and others Used when citing a list of more than three authors to substitute for all the authors' names except the first. ibid. [ibidem] in the same place Refers to immediately preceding footnote op. cit. [opere citato] in the work cited Examples of ibid. and op. cit. 4. R. Poirer, "Learning physics," (Academic, New York, 1993), p. 4. 5. Ibid, p. 9. 6. T. Eliot, "Astrophysics," (Springer, Berlin, 1989), p. 141. 7. Poirer, op. cit., p. 89. 8. Eliot, op. cit., p.148.
Today, many omit op. cit. and ibid. because they add no real information.
In the case where the same author has written more than one book, use author and date -- e.g., Eliot, 1997, p. 213, to distinguish from book by the same author in (say) 1994.