Use secondary sources sparingly; you should always try to locate the original source of information which is cited in a work that you have read. This is, however, not always possible; sometimes the original work is out of print, unavailable through your usual sources or not available in English.
If you read an article or book which cites some information that you want to cite, always refer to the source where you found the information, not the original source.
If you read an article by Hulya Ipek, in which she cites information from a previous study by Lantolf and Thorne, and you wish to refer to this information in your assignment:
• You would acknowledge Lantolf and Thorne in the text:
Lantolf and Thorne’s study found that “what one can do today with assistance is indicative of what one will be able to do independently in the future..." (qtd. in Ipek 158).
• In your Works Cited list at the end of your assignment, your entry would be a reference for Ipek's article because that is where you sourced the information:
Ipek, Helya. "Comparing and Contrasting First and Second Language Acquisition: Implications for Language Teachers." English Language Teaching, vol. 2, no. 2, 2009, pp. 155-63.