In this lesson, the focus is on understanding the use of negative particles in Arabic, particularly the particle لم (lem), which indicates the past negative and is used with the subjunctive. The teacher explains the differences between various negative particles such as لا (la) for the present indicative, ما (ma) for both present and past tenses, and لن (lan) for the future negative. Additionally, there is a brief discussion on ليس (laysa) and غير (ghayr), which are used with nouns or adjectives rather than verbs.