The aims can be flexible and dependable on several things like your students' level and interests, syllabus, teaching method, and some others. It is recommendable to write the aims before starting a lesson plan to make sure what learners are able to do at the end of the lesson.
Yo have seen many times that the aims start like this:
By the end of the lesson, learners will be able to....
At the end, students will be able to......
Learners will be better able to....
Many teachers around the world use the SMART goal setting model:
Smart
Messurable
Attainable
Relevant
Time-bound
However, SMART model is not the only one, some teachers use the following criteria to write lesson aims.
1. Required by syllabus/curriculum
2. Learned-Centred
3. Specific
4. Observable