までに

までに means "by": it sets a deadline, and the action happens once, at some point before it. It follows a time noun, or a verb in the dictionary form for "by the time".

までに and まで

まで is "until": the action goes on the whole time up to the limit. までに is "by": the action is done once, before the limit.

Anything that happens once – arriving, handing in, deciding – needs までに.

• × 金曜日までレポートを出してください → 金曜日までにレポートを出してください

Before a verb

The verb stays in the dictionary form even when the sentence is in the past.

• × 友達が来たまでに → 友達が来るまでに