about german language
what's the different :
a. vollkommen with perfekt , why it must be "der vollkommen ist ?
b. i think er , ihrem , and ihn have a same meaning but why in statement of "er macht meinen tag besser als gestern " use er ? and "er ist mein zukunftiger ehemann " use er ist mein and " ich liebe ihn sehr " use ihn ?
c. what the different of zukunftiger and zukunftigen ?

Question

Answer

(you probably should have asked this question for German language, not English)

No,  "er" "ihn" and "ihrem" don't have the same meaning.

"er"  means "he"  (nominative)
  "ihn" means "(to) him" (accusative)
  "ihrem"  means "her" (if the next noun is masculine/neuter , and is dative case)