[Maxful]

Also, I am confuse by these sentences.



1. 書類の重要なポイントに印を付ける
mark highlights of a document


2. ×印を付ける
mark a cross


3. カレンダーにその日の覚えとして印を付ける
mark the date on the calendar


4. ペンで○や×を付ける
mark/draw circles and crosses with a pen



First of all, I would like to know why there isn't a need to include "に" before "印" for number 2 and 3?

Secondly, why was "に印" excluded from number 4?