Rule 1: 1. The jussive endings
Where the indicative ends in -u, the jussive has sukūn: يَسْجُدُ becomes يَسْجُدْ, أَسْجُدُ becomes أَسْجُدْ. Where the indicative ends in nūn (ـِينَ، ـَانِ، ـُونَ) the nūn is dropped, and ـُوا takes the silent alif. The two feminine plurals keep their ـْنَ. When a jussive ending in sukūn is followed by a word beginning with hamzat al-waṣl (the article, اِبْن, an imperative), the sukūn becomes kasra to avoid two vowelless letters meeting: لَمْ يَدْخُلِ الْمَسْجِدَ.
يَسْجُدُ، يَسْجُدْ
yasjudu, yasjud
he prostrates; (jussive) he prostrate
تَسْجُدِينَ، تَسْجُدِي
tasjudīna, tasjudī
you (f.) prostrate; (jussive)
يَسْجُدُونَ، يَسْجُدُوا
yasjudūna, yasjudū
they (m.) prostrate; (jussive)
Silent alif after ـُوا.
يَسْجُدْنَ، يَسْجُدْنَ
yasjudna, yasjudna
they (f.) prostrate; (jussive, unchanged)
لَمْ يَدْخُلِ الْمَسْجِدَ.
lam yadkhuli l-masjida
He did not enter the mosque.
Sukūn becomes kasra before the article.