Does the lesson start when the teacher gets into the classroom? Does it start right after the teacher takes the attendance or when the students open their books?  When does the lesson start?             

The lesson starts when the teacher begins to think about the content of his/her lesson and how it relates to his/her students’ age group, interests, skills and learning styles. Exploiting the materials according to the very needs and the background knowledge (or schemata) of the particular group are very important.

Teacher has to consider the content of the lesson in relation to the time fit within the program as well. How much do the students already know about the content and what needs to be exploited in the course book accordingly have to be considered. For example, do the students know anything about the main theme of the lesson? How much do they already know about the language point? How would this affect their learning?

Say, the focus is Unreal Conditionals; teacher should think and evaluate what learning difficulties or misconceptions may occur while introducing the language point. How the language point in question is represented in L1 and whether or not this may cause any confusion should also be thought about while planning the lesson.

In other words, teacher has to think about the lesson objectives in relation to his/her students. What will the students be able to do at the end of this lesson that they weren’t able to do before?

Of course, this is not enough. You have to think a couple of more things: Are the lesson aims realistic, are they conceptually offering enough challenge to the students but not overwhelming them. Are they clear, are they assessable? (i.e. would there be tangible learning outcomes that would allow the teacher to get some feedback on how much the lesson introduced is acquired?) How much more exercises do the students need to improve the language point?

Is that all? Of course not. And remember, teacher has not stepped into the lesson yet:) Now that the teacher thought about his aims and the relevance of the content to his/her students, it is time to think about what activities will allow the learners to reach these aims. How are these activities going to engage the learners into a meaningful learning environment? What kind of interaction patterns will be implemented?  Are the activities coherently linked? What about timing of the activities?

What about vocabulary? Teacher has to go through the material and see if there are words that require pre-teaching. If so, how is he/she going to teach them, how much time should be allocated to this stage?

The lesson has already started and the teacher has not entered the class yet..