Rule 1: 1. إِلَّا 'except', and لَا … إِلَّا 'only'
إِلَّا means except. Its most useful pattern is after a negative verb, where not … except amounts to English only: مَا سَمِعْتُ إِلَّا صَوْتًا I heard only a voice. The perfect is negated with مَا, the imperfect with لَا. The noun after إِلَّا takes the case it would have had without the negative, here the accusative of the object.
مَا رَأَيْتُ فِي الْجِبَالِ إِلَّا الصُّخُورَ.
mā raʾaytu fi-l-jibāli illa-ṣ-ṣukhūra
I saw nothing in the mountains except the rocks.
لَا يَسْمَعُ الْوَلَدُ إِلَّا صَدَى صَوْتِهِ.
lā yasmaʿu-l-waladu illā ṣadā ṣawtihi
The boy hears only the echo of his voice.
مَا جَاءَ إِلَّا عَمِّي.
mā jāʾa illā ʿammī
Only my uncle came.
The subject after إِلَّا is nominative.
لَا أَعْرِفُ هُنَا إِلَّا هٰذَا الطَّرِيقَ.
lā aʿrifu hunā illā hādha-ṭ-ṭarīqa
I know only this road here.