when used as a noun, 'lesson' refers to a section of a course that provides instruction or teaching.
it must be preceded by a determiner, such as 'the' or 'a'; it can be modified by an adjective; and it can replace the answer to a question about what is being taught or learned.
1. I learned a valuable lesson from my teacher today.
2. The lesson was difficult to understand, but I tried my best.
3. Our class held a pleasant discussion about the lesson this afternoon.
when used as a noun, it is important to note that 'lesson' can only be used to refer to a valid learning or teaching situation, and it cannot refer to any kind of educational instruction, such as advice or scolding. Additionally, 'lesson' should never be used directly after a verb such as 'teaching' or 'learning', as the verb should be followed by a gerund (e.g. 'I am learning a baking lesson' should be changed to 'I am learning how to bake').