I need someone to explain me why "nicht" goes at the end of the sentence here
Not 100% sure abt this, but let me give it a try: nicht comes after the word it negates if that word is a verb.
Eg: "Ich laufe nicht." I am not walking.
If the word it negates is not a verb, it retains its position before the word.
Eg: "Es ist nicht weiß." It is not white.
In case of the above sentence, the nicht needs to come after the verb.
Eg: "Ich mag nicht ihren Stuhl." I do not like her chair.
However German also accepts the nicht after the accusative clause (ihren Stuhl) - for emphasis seems to be the reason.
Eg: "Ich mag ihren Stuhl nicht." I do not like her chair.
I hope this helps. Again please correct me if there's something wrong :)
Nicht goes before the word it negates when it's not a verb. It is situated in the last place when negating a verb, however something modifying and completes the verb thereafter should be put in the last place. In the second case, nicht appears to be preceding the retinue of the verb. That's because the verb altogether with auxiliary verbs and prepositional clauses and pastparticiples etc., is considered as a whole predicate.
"because ihren is in plural"
There's nothing here to show a plural; you're likely confused because the tables for the feminine third person singular (she) and the third person plural (they) are ALMOST EXACTLY THE SAME, except for the dative (indirect object), where "her" is "ihr" (no comment) and "them" is "ihnen".
So doch, "her chair" is "ihren Stuhl" and "their chair" is "ihren Stuhl".
Why didn't Martin Luther fix it when he had a chance, that's what I want to know. Where is L.L. Zamenhoff when you need him?