أَعْنِي الْحَرْفَ وَالْأَمْرَ الْحَاضِرَ وَالْمَاضِيَ، نَحْوُ: «زَيْدٍ» فِي «قَامَ زَيْدٌ»، لَا «زَيْدٌ» وَحْدَهُ، لِعَدَمِ التَّرْكِيبِ، وَلَا «هَؤُلَاءِ» فِي «قَامَ هَؤُلَاءِ»، لِوُجُودِ الشَّبَهِ، وَيُسَمَّى مُتَمَكِّنًا.
Translation: By inherently fixed forms the author means particles, commands directed to someone present, and past-tense verbs.
For example, “Zayd” in “Zayd stood” is a declinable noun. “Zayd” standing alone has not been used in the grammatical construction described here.
Likewise, “hāʾulāʾi” in “These people stood” is excluded owing to resemblance to inherently fixed forms.
Such a declinable noun is called mutamakkin.
The source wording is preserved, with vowel marks added as reading aids.