illī is a X of ille declension. illī is singular dative masculine form of ille.
In the first sentence what we should say instead of X?
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I would simply say: "Illi is an inflected form of ille. Illi is the singular dative masculine form of ille." (I am not sure what the role of "declension" in your example is. I found it more natural to leave it out.) In all generality, I would just use the expression "inflected form". In a more specific case, you can consider "oblique case", but that only applies to case inflection.