must fill in the application from in a black pen.分析

同意楼上的说法,正确的应该是you mush fill in the appication form in a black pen.
可以用with a black pen.再多几个例句如下,我个人更建议用 with a black pen.
1.Please fill in the blanks with a black pen.请用黑墨水笔填写空格.
2.Please fill in the form clearly with a black or blue ink pen此表请用黑色或蓝色水笔填写