'in' is correct. Using 'about' is incorrect - it should not be in the sentence. 'We will be discussing grammar in the next lesson.' is how it should be.

Hi friends, Which is the correct sentence? (in or on) 1. We will be discussing about grammar in the next lesson. 2. We will be discussing about grammar on the next lesson. Hi Niwantha, Sentence #1 is grammatically correct. We can use the preposition "in" to describe a period or duration of time. Example: In the next lesson, we will be discussing on gerunds. / The flowers are in full bloom in summer. Hope this helps. Cheers, Lance