Translation:Whoever is not for us is against us.
I am not very good with terminology, but I think "Wer nicht fur uns ist" is a subordinate sentence, as in, it wouldn't make sense on its own; so, whenever you have a sentence that doesn't make sense on its own, you need to put the verb at the end. Someone please correct me if I'm wrong :)
A relative clause is normally classed as a specific type of subordinating clause. The easiest way to tell if something is a subordinating clause is to see if it stands on its own. "Who is not for us" does not stand on its own (except as a question), so it must be subordinate.
And is grammatically wrong in English and German. If you would replace it with "he" then "who" is appropriate. if you would replace using "him" then "whom" is appropriate. (eg he is not for us so use who is not against us).
Answering your question "Whom" can be "wen" (accusative) or "wem" (dative) "wen hat du getroffen?" and "wem hat du dieses Buch gegeben?" (to whom did you give this book)
"And is grammatically wrong in English and German. If you would replace it with "he" then "who" is appropriate. if you would replace using "him" then "whom" is appropriate."
...then it's not grammatically incorrect to use 'whom' in English? It sounds to me like whether or not Who or Whom is used depends entirely on context outside of the content of the sentence in question.
"Who" and "whom" have different grammatical functions.
"Who" is a subject:
Who is talking?
She is a woman who has done many things.
Everyone who wants to come on the trip should sign up today.
"Whom" is an object, used to follow prepositions or to receive the action of a verb (when you would use an accusative/dative form in German, although English uses the same forms for accusative/dative):
To whom should I address this letter?
To whom it may concern:
In speech, however, many people just use "who."
No, my point was it was grammatically incorrect to use "whom" in your example "to whom did you give this book" is perfectly acceptable.
It is true however than many people, including many journalists, have dropped "whom" altogether and use only "who" in all cases. However using "whom" when it should be "who" is always bad grammar.
"Wer nicht für uns ist, ist gegen uns" "Wer" introduces the subordinate clause "Wer nicht fuer uns ist". Since it is a subordinate clause, the verb is kicked to final position. I am not sure why "nicht" is before "fuer", but I can never figure out the placement of "nicht" anyway. The subordinate clause "Wer nicht fuer uns ist" takes up the first position of the sentence (and acts as a subject, I believe), so the verb "ist" follows. "Gegen" and "uns" are then placed as they usually would be.
"Wer" is like "who" in English. "Who" can be used to either introduce a question ("Who is that") or to introduce a subordinate clause ("Whoever he is, he shall fail."). In German, "wer" can do the same, it can introduce a question or it can introduce a subordinate clause. Also, "wer" (as "who" in English) introduces a special sub-type of subordinate clause called a relative clause (https://en.wikipedia.org/wiki/Relative_clause). There are a few ways to introduce a relative clause. One way would actually be to use the relative pronouns (der, das, die, and die and their inflections). The question words in German ("wer", etc.) are normally only used to introduce a relative clause which has an antecedent from another sentence. This might explain it better than I can: http://www.lsa.umich.edu/german/hmr/grammatik/relativsaetze/relative.html