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I'd suggest that the three factors below are the main aspects that should be considered when planning a lesson, but feel free to dissagree with me!
1. Who are the students? (level of English, 1st language (and 2nd etc.) age, gender, occupation, socio-cultural background, attitudes to formal learning etc.)
2. What do they want to learn, and what do they need to learn? (The answers to these 2 questions may be different, and your decisions about how (and whether) to give them what they want and what they need will be obviously be influenced by factors 1 and 3)
3. Restrictions (How long is the lesson, and is it part of a course? Is there a fixed syllabus? What resources are available to you (material and equipment)? Are there topics you cannot or should not cover? )