First of all, it needs to be clear that there is no uniform regulation on the citation of documents in English papers, whether in or out of the period. Different publishers, academic journals and teachers may have different requirements. Generally speaking, publishing institutions, including APA, MLA and University of Chicago Press, all have norms and standards for paper writing, but they also have different requirements for citation.

For papers in APA format, in most cases, quotations should be placed in brackets at the end of sentences. For example, "According to Smith (20 10), research shows that …" or "Research shows that … (Smith, 20 10)" are all correct. The first one has brackets after "Smith", and the second one puts the quoted information in the whole sentence. It should be noted that if the referenced file itself contains a period, it should be placed directly in parentheses.

For papers in MLA format, generally speaking, the cited documents should be placed at the end of the whole sentence. For example, "research shows that … (Smith 10)." Here, "10" refers to the page number of the content cited in the document, which can be a specific page number or chapter number.

Chinese papers generally adopt GB/T77 14-20 15 standard. According to this standard, in Chinese papers, the cited documents should generally be placed at the end of the whole sentence and enclosed in Chinese brackets. For example: "According to Li Ming's research results (20 10), it shows ……" or "shows ………" (Li Ming, 20 10). It should be noted that in the citation of Chinese-English mixed literature, Chinese brackets should appear in pairs, that is, the left and right brackets cannot be mixed.

To sum up, we need to judge and deal with the cited documents in or out of the period according to specific academic norms. When writing, you should carefully read the requirements published by relevant academic journals or teachers, and quote them in strict accordance with the norms. This can not only improve the quality of the paper, but also enhance the author's academic reputation and provide more powerful support for future research.