までに・まで（名詞）: the difference
までにN4
Meaning
By ~, By the time that ~, Before ~
Past papers
Tested 2 times in past papers
Example
明日までに電話をください。
Please call by tomorrow.
まで（名詞）N5
Meaning
Until, Till, To
Past papers
Tested once in past papers
Example
どこまで行く？
How far are you going?
How to tell them apart
「まで」: it goes on without a break until that time, and a continuing action follows: 「5時まで勉強する」.
「までに」: it gets done at some point before that deadline, and a one-time action follows: 「5時までに宿題を提出してください」 (Please hand in your homework by five).
In an exam, check whether the verb at the end is a continuing action or a one-time action, and you can choose correctly.