How to think about "The language"

Of course you can predict what forms and what vocabulary they might use but despite your best efforts, in a Task-based classroom you just never know. I know, it´s easier to do it the other way around: first give them the forms and then ask students to practice them. But an advantage from a planner´s perspective is that by in order to predict what students will do or say during the task(s), you have to visualize it or even do it yourself. This means you´ll be in a better position to determine the level of enjoyablity (which is key) as well as find any "cracks" in your planning. Meanwhile, if you plan the forms you´re going to teach and then think of a "communication activity" to practice it, or just blindly follow a textbook, what happens is you probably won´t tend to "think through" the lesson. Instead, you rely on the material´s infallibility--something which in my experience leads to disappointing, or even disastrous results.

So thinking through a lesson and what students will do leads you to consider not only grammar and vocabulary that might come up but also grammar and vocabulary that won´t come up. In other words, grammar and vocabulary they don´t know yet but which would be helpful for them to know. This is where you´ll want to focus your efforts in planning. Decide what new language you´ll present (of course, after the task) and how you´ll do it.