Rule 1: 1. Imperatives with an object pronoun
The object pronouns you know (ـهُ, ـهَا, ـهُمْ, ـنِي and the rest) attach directly to the imperative. Two spelling points: the silent alif written after ـُوا is dropped once a suffix follows (اِفْتَحُوا but اِفْتَحُوهُ), and a final sukūn stays as it is (اِفْتَحْهُ, iftaḥhu).
اِفْتَحِ الْبَابَ. اِفْتَحْهُ.
iftaḥi l-bāba. iftaḥhu
Open the door. Open it.
اِغْسِلِي الْمَنَاشِفَ. اِغْسِلِيهَا.
ighsili l-manāshifa. ighsilīhā
Wash (f.) the towels. Wash them.
اِحْمِلُوا الصُّنْدُوقَ. اِحْمِلُوهُ.
iḥmilu ṣ-ṣundūqa. iḥmilūhu
Carry (pl.) the box. Carry it.
The alif of اِحْمِلُوا disappears before ـهُ.
اُكْتُبْ لِي رِسَالَةً وَلَا تَتْرُكْنِي بِلَا خَبَرٍ.
uktub lī risālatan wa-lā tatruknī bi-lā khabarin
Write me a letter and do not leave me without news.
ـنِي 'me' after the jussive of the prohibition.