Good aims are clear and not only to you.  Learners, too, must know what they are trying to achieveachievedExtensionfocusPresentationprogressrealisticreviseRevisiontime.
Too many aims will mean that the lesson loses achieveachievedExtensionfocusPresentationprogressrealisticreviseRevisiontime.
A good lesson plan will also make it clear how the aims will be achieveachievedExtensionfocusPresentationprogressrealisticreviseRevisiontime. 
Aims need to be achieveachievedExtensionfocusPresentationprogressrealisticreviseRevisiontime in terms of the level of the learners, the amount of material and the achieveachievedExtensionfocusPresentationprogressrealisticreviseRevisiontime available.
If aims are too ambitious, learners will be discouraged because there will be no sense of achieveachievedExtensionfocusPresentationprogressrealisticreviseRevisiontime.
Lessons come in different sorts and will need different aims:
achieveachievedExtensionfocusPresentationprogressrealisticreviseRevisiontime lessons in which a new skill or structure is met for the first time
achieveachievedExtensionfocusPresentationprogressrealisticreviseRevisiontime lessons in which there is nothing completely new but the learners need more exposure to the form or the skill
achieveachievedExtensionfocusPresentationprogressrealisticreviseRevisiontime lessons in which the aim is to broaden, widen or deepen the learners' abilities in some way
Revision plus extension lessons in which the learners first achieveachievedExtensionfocusPresentationprogressrealisticreviseRevisiontime what they know and then go on to extend it and apply it