Rule 1: 1. The prohibition: لَا تَفْعَلْ
To tell someone not to do something, put لَا before the second-person imperfect and drop its final vowel to sukūn: تَنْظُرُ you look, لَا تَنْظُرْ do not look. For a woman the ending ـِينَ loses its نَ: لَا تَنْظُرِي; for a group ـُونَ becomes ـُوا: لَا تَنْظُرُوا. In a hollow verb the long vowel shortens when the sukūn arrives: تَخَافُ you fear, لَا تَخَفْ do not fear; تَقُولُ, لَا تَقُلْ. Compare the plain negative لَا تَنْظُرُ you do not look, which keeps its ḍamma.
لَا تَخَفْ مِنَ الْكَلْبِ، إِنَّهُ صَغِيرٌ.
lā takhaf mina-l-kalbi, innahu ṣaghīrun
Do not be afraid of the dog; it is small.
خَافَ مِنْ: to be afraid of.
لَا تَنْزِلْ يَا بُنَيَّ!
lā tanzil yā bunayya
Do not come down, my little son!
لَا تَنْظُرِي إِلَى الشَّبَكَةِ يَا أُخْتِي.
lā tanẓurī ila-sh-shabakati yā ukhtī
Do not look at the net, my sister.
Feminine: the نَ of ـِينَ is dropped.
لَا تَضْحَكُوا مِنَ الْفُقَرَاءِ.
lā taḍḥakū mina-l-fuqarāʾi
Do not laugh at the poor.
Plural: ـُونَ becomes ـُوا.
لَا تَقُلْ هٰذَا الْكَلَامَ.
lā taqul hādha-l-kalāma
Do not say these words.
Hollow verb shortened: تَقُولُ, لَا تَقُلْ.