Rule 1: 1. The three moods
Only the ending changes; the prefix and the stem stay exactly as in the indicative. The mood is decided by what stands before the verb: a plain statement takes the indicative, the particles of this lesson take the subjunctive, and the particles of lesson 71 take the jussive.
يَرْجِعُ الْمُسَافِرُ غَدًا.
yarjiʿu l-musāfiru ghadan
The traveller returns tomorrow.
Indicative: final -u.
أُرِيدُ أَنْ يَرْجِعَ الْمُسَافِرُ غَدًا.
urīdu an yarjiʿa l-musāfiru ghadan
I want the traveller to return tomorrow.
Subjunctive after أَنْ: final -a.
يَرْجِعْ
yarjiʿ
(jussive form: no final vowel)
You will meet the jussive in lesson 71.
تَرْجِعِينَ، أَنْ تَرْجِعِي
tarjiʿīna, an tarjiʿī
you (f.) return; that you (f.) return
Where the indicative ends in nūn, the subjunctive drops it.