Like an abstract in a published research article, the purpose of an article summary is to give the reader a brief, structured overview of the study. To write a good summary, identify what information is important and condense that information for your reader. The better you understand a subject, the easier it is to explain it thoroughly and briefly.
Write a first draft. Use the same order as in the article itself. The number of suggested sentences given in parentheses below is only a rough guideline for the relative length of each section. Adjust the length accordingly depending on the content of your particular article.
For the first draft, focus on content, not length (it will probably be too long). Condense later as needed. Try writing about the hypotheses, methods and results first, then about the introduction and discussion last. If you have trouble on one section, leave it for a while and try another.
Edit for completeness and accuracy. Add information for completeness where necessary. More commonly, if you understand the article, you will need to cut redundant or less important information. Stay focused on the research question, be concise, and avoid generalities. The Methods summary is often the most difficult part to edit. See the questions under 'Reading interactively' to help you decide what is important to include.
Edit for style. Write to an intelligent, interested, naive, and slightly lazy audience (e.g., yourself, your classmates). Expect your readers to be interested, but don't make them struggle to understand you. Include all the important details; don't assume that they are already understood.