All of the rules with respect to punctuation, grammar, and mechanics that apply to quotations from primary texts also apply to quotations from critical, or secondary texts. Quotations from secondary texts can serve a variety of functions in an essay.
Example
Andremes Kudavagnan argues that the most prominent feature of Toronto Suburbanist poetry is its reliance on mall imagery. According to Kudavagnan, "direct references to the mall, the use of the mall's features in similes and metaphors, and the peculiar language of the suburban mall permeate the best works of the Suburbanists" (3456). Kudavagnan compiles an impressive list to prove his case, and his examples do not need repeating here. There are, however, distinctions that need to be made between the way mall imagery is manipulated by the Suburbanists whom Kudavagnan considers.
Note, first of all, how long this example is. As in the case of quotations from primary texts, quotations from secondary sources do not stand on their own. They need to be introduced. They need a context to be made meaningful. The quotation in this example is used to suggest a hypothesis - - Suburbanist poets use mall imagery - - that the essay's author accepts, and wishes to refine further. Note that the quotation from Kudavagnan does not prove that suburbanists use mall imagery. If the writer thinks this is a controversial point that needs proving, she or he will have to turn to the primary texts to do so.
There are many different responses to the Toronto Suburbanist movement. Shaughnard Phileets calls the Suburbanists "the worst of the worst, the nadir, the pits of modern literature" (3), while Tracia Ozarway argues "The Suburbanists are to twentieth century art what the glaziold sculptors were to craft of the ampelic period" (Suburbanists 2).
The quotations are not proof of anything about the Suburbanists. They are proof that there has been a range of response to the Suburbanists.
While it should now be clear that the Suburbanists use mall imagery to resist notions of Urban superiority and Suburban alienation, it should be noted that other elements of their poetry contributed to the same end. Rewodd Belhut argues that "the Suburbanists' metrics make a claim for the greater integrity of their culture" (17), and, while criticizing the arrogance of the Suburbanists' ideological position, Robayne Macett points out that "the Suburbanists skillfully employ allusions to undercut Urban and Rural aesthetics (482).
This is the closest that secondary sources come to being taken as "proof" of something in the primary texts. The essayist is not primarily concerned with proving the claims of Macett and Belhut, so she or he has decided to let their comments stand as evidence. If their claims were central to the essay's thesis, evidence from the texts would need to be brought forward.