Courses on Khan Academy are always 100% free. Start practicing—and saving your progress—now: We use the perfect progressive aspect to talk about an action that was once ongoing, but has since completed, like "I had been waiting for twenty minutes." Practice this yourself on Khan Academy right now: Watch the next lesson: Missed the previous lesson? Grammar on Khan Academy: Grammar is the collection of rules and conventions that make languages go. This section is about Standard American English, but there's something here for everyone. About Khan Academy: Khan Academy offers practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom. We tackle math, science, computer programming, history, art history, economics, and more. Our math missions guide learners from kindergarten to calculus using state-of-the-art, adaptive technology that identifies strengths and learning gaps. We've also partnered with institutions like NASA, The Museum of Modern Art, The California Academy of Sciences, and MIT to offer specialized content. For free. For everyone. Forever. #YouCanLearnAnything Subscribe to Khan Academy’s Grammar channel: /频道 Subscribe to Khan Academy: "