The difference between لم، لن، ما، ليس، لا in arabic msa ‎لم lam is used to negate the past using the present tense. Ex: i didnt do that! ‎أنا لَمْ أَفْعَلْ ذلك ‎‏ It has to include the verb in the present tense. I didnt do the homework. انا لم أفَعَلْ الفَرْضَ. ‎لن lan enters the presnet tense verbs but negates it in the future. Ex: i will not go to the meeting tomorrow. لن أَذْهَبَ إلى الإِجْتِماعِ غداً. I will not do this. لن أَفْعَلَ هذا ‎لا la enters both nouns/names(ism) and verbs and negates them. Ex: don't touch this painting. لا تَلْمُسَ هذهِ اللَوْحَة I don't like this. انا لا أحب هذا ‎ما Ma negates a verb whether it was past or present. Ex: the student didn't go out quickly. الطلبُ ما خَرَجَ سريعا He doesn't help the needy. هو ما يُساعدُ المحتاجَ ‎ليس Laysa negates a nominal sentence. The weather is not hot today. ‎ليس الطَقْسُ حاراً اليوم I am not a doctor. انا لَسْتُ طَبيباً The boy is not happy. ليس الولد مسرورا