以前【いぜん】

以前 means "before". On its own it is an adverb, "previously", and after a date or a noun it means "up to and including that point". Its N1 use is emphatic: after a noun or a dictionary-form verb, きごう〜いぜん以前のもんだい問題だ or きごう〜いぜん以前に says the thing just named is not even the issue yet, because something more basic is wrong.

"Before even": 以前の問題 and 以前に

The noun or verb before 以前 is the topic someone wants to discuss. The rest of the sentence names the more basic failure that comes first, and the tone is critical or exasperated.

まえに前に cannot replace 以前 here. ×マナー前の問題だ is not Japanese, and 議論する前に only gives the order of events, without the criticism.

"Previously" and "up to"

As an adverb, 以前 means "before now", like まえに前に but a little more formal. いぜん以前の before a noun means "former".

After a date or an event, 以前 means "up to then" and includes the point itself, the opposite of いこう以降.

To say "before doing" in the ordinary sense, use きごう〜まえに前に: しゅっぱつ出発するまえに前に. A verb with 以前に reads as the emphatic "before even".