An-Nisāʾ: 13
"These are the limits of Allah..."
"These"
This is a reference to the rulings mentioned regarding orphans, bequests, and inheritance. He called them "limits" (ḥudūd) because religious laws are like boundaries set and fixed for those who are legally responsible (mukallafīn); it is not permissible for them to transgress or step beyond them into what they have no right to.
"He will admit him"
It is recited with both the yāʾ (He will admit) and the nūn (We will admit). The same applies to the phrase "He will admit him into a Fire."
It has been said that "He will admit him" (yudkhilhu) and "abiding therein" (khālidīna) are based on the wording of "whoever" (man) and its meaning.
The words khālidīna (abiding) and khālidan (abiding) are in the accusative case (naṣb) as a state (ḥāl). If you ask: "Is it permissible for them to be adjectives (ṣifāt) for 'Gardens' and 'Fire'?" I would say: No, because they are derived from something other than what they refer to.
Therefore, a pronoun is necessary, as in your saying: "abiding therein" (khālidīna hum fīhā) and "abiding therein" (khālidan huwa fīhā).