I don't understand vs. I do not understand

[May Thant Zin]

Test No. errors/elem-4 "Present Tense", question 5

Sometimes I not understand what the teacher says to me.

(a) not understand
(b) says
(c) to

Test No. errors/elem-4 "Present Tense", answer 5

Sometimes I don't understand what the teacher says to me.

Correct entry: don't understand
The error was: (a) not understand

You have found the error but your entry is incorrect.
Sometimes I do not understand what the teacher says to me.
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Why is my answer ''do not understand" is incorrect? "Don't understand" is the short form of it, isn't it? Thanks.

[Mister Micawber]

.
Yes, you are correct. I'm sure that Alan will adjust the machine to accept that answer as well.
.
_________________
Native English teacher at Mister Micawber's

[Narayanan Krishnaswamy]

Many mistake that 'don't' is spoken form and 'do not' written.
MS Word corrects 'don't' to 'do not'.

"short form of do not:
Don't do that - it hurts!" Cambridge.
Usage - do's and don'ts
So according to me,the better form is 'don't' and not 'do not,' unless you want to emphasize the negative.
Narayanan.