"Their father is strict."
Translation:Pater eorum est severus.
Oh! sorry--and thanks for clarifying the question!!
There is not a huge difference between eōrum and illōrum , except that illōrum is somewhat more 'emphatic' and can be used in a couple of special ways.
They are alike in meaning "of them," "their" (referring to some masculine plural or neuter plural group under discussion).
eōrum is the genitive plural masc/neuter form of the ordinary, regular 3rd person pronoun (is, ea, id , "he, she, it").
illōrum is the genitive plural masc/neuter of the demonstrative pronoun/adjective (ille, illa, illud , "that"). So, in addition to meaning "of those [men/things under discussion]", which is not dramatically different from "of them", it can mean: "of those FAMOUS [men/things]" , or "of the FORMER [men/things under discussion]" , with an implied contrast to hōrum (the gen pl m/n of hic, haec, hoc , "this") meaning, "of the LATTER."
It's clear from the Romance languages that the ille, illa, illud word was eventually used so widely that it seems to have replaced is, ea, id as a 3rd person pronoun and as the source of the definite article (not used in Latin).
"eōrum" is the genitive plural masculine (or neuter) form of the pronoun is, ea, id ("he, she, it"). -ōrum is the gen. pl. masc/neuter 2nd decl. ending. Translate it as "their" or "of them".
All the genitives plural have some kind of "m" ending: -ārum, -ōrum, -um/-ium, -uum, -ērum (that's for decl. 1-5 respectively).
Apparently not; the Oxford Latin Dictionary has these definitions for strictus, a, um (the past participle of stringere , which means, among other things, to draw a sword): "compact, well-knit; closely packed, dense"; "terse" (of writings); "rigorous" of a judge or a law. One would guess that the meaning of "strict" in English, referring to people's conduct/attitudes, comes from this meaning.