Rule 1: 1. Compound verbs: each weak radical keeps its own rule
Five combinations are common. أَتَى 'to come': hamza first + ي third. وَقَى 'to protect': و first + ي third. رَأَى 'to see': hamza second + ي third. رَوَى 'to relate' and أَوَى 'to take shelter': و second + ي third; here the middle و is treated as a strong consonant and only the final ي is weak. حَيِيَ 'to live': ي second and third. In every case, conjugate the verb as defective and then apply the rule of the other weak radical.
أَتَى + يَأْتِي = أَكَلَ يَأْكُلُ + رَمَى يَرْمِي
atā, yaʾtī
'to come': the hamza of أَكَلَ, the ending of رَمَى
First person آتِي, like آكُلُ.
وَقَى + يَقِي = وَعَدَ يَعِدُ + رَمَى يَرْمِي
waqā, yaqī
'to protect': the و drops as in يَعِدُ, the ending is that of يَرْمِي
رَوَى يَرْوِي
rawā, yarwī
to relate, transmit
The و stays: only the final ي is weak.
حَيِيَ يَحْيَا
ḥayiya, yaḥyā
to live
Defective with kasra in the perfect, like لَقِيَ يَلْقَى.