When I borrow repeatedly from the same source, how should my parenthetical citations appear?

You may be able to combine citations in a single parenthesis after a sequence of borrowings:

Romeo and Juliet presents an opposition between two worlds: "the world of the everyday . . . and the world of romance." Although the two lovers are part of the world of romance, their language of love nevertheless becomes "fully responsive to the tang of actuality" (Zender 138, 141).

Here it is clear that the first page number in the parenthesis must apply to the first quotation and the second number to the second quotation.

But suppose you decide to break the first quotation into two parts. Then the parenthetical citation will be ambiguous, because three quotations will be followed by two numbers. It will not be clear how the page numbers should be matched to the borrowings. In that case, the citations should be separated. You can use another technique for making citations more economical--not repeating what is understood:

Romeo and Juliet presents an opposition between two worlds: "the world of the everyday," associated with the adults in the play, and "the world of romance," associated with the two lovers (Zender 138). Romeo and Juliet's language of love nevertheless becomes "fully responsive to the tang of actuality" (141).

The second parenthetical citation--"(141)"--omits the author's name. This omission is acceptable because the reader will conclude that the author must be Zender. No other understanding is possible. However, if you include material from a different source between the two borrowings from Zender, you must repeat this author's name in the second citation: "(Zender 141)."

A third technique is to define a source in the text at the start:

According to Karl F. Zender, Romeo and Juliet presents an opposition between two worlds: "the world of the everyday," associated with the adults in the play, and "the world of romance," associated with the two lovers (138). Romeo and Juliet's language of love nevertheless becomes "fully responsive to the tang of actuality" (141).

This technique can be useful when an entire paragraph is based on material from a single source. When a source is stated in this way and followed by a sequence of borrowings, it is important to signal at the end of the borrowings that you are switching to another source or to your own ideas. For example:

According to Karl F. Zender, Romeo and Juliet presents an opposition between two worlds: "the world of the everyday," associated with the adults in the play, and "the world of romance," associated with the two lovers (138). Romeo and Juliet's language of love nevertheless becomes "fully responsive to the tang of actuality" (141). I believe, in addition, that . . .

(The quotations in the above examples are taken from Karl F. Zender, "Loving Shakespeare's Lovers: Character Growth in Romeo and Juliet," Approaches to Teaching Shakespeare's Romeo and Juliet, ed. Maurice Hunt [New York: MLA, 2000] 137-43.)