Here هُ، هِ، هْ are possible realizations of a pronoun, not rules for every lexical hāʾ. In رَسُولُهُ, the last hāʾ means “his”; in مِنْهُ, it means “from him/it”.
Look at the sound immediately before the pronoun hāʾ. A kasrah or an unvowelled yāʾ may determine its vowel. Do not classify merely by seeing the letter ه.
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Identify the pronoun hāʾ
Rules of Hāʾ al-Ḍamīr: Vowels, Ṣilah and Exceptions
Here هُ، هِ، هْ are possible realizations of a pronoun, not rules for every lexical hāʾ. In رَسُولُهُ, the last hāʾ means “his”; in مِنْهُ, it means “from him/it”.
Look at the sound immediately before the pronoun hāʾ. A kasrah or an unvowelled yāʾ may determine its vowel. Do not classify merely by seeing the letter ه.