An affirmative sentence must, in the negative sentence, change “must” to “cannot but” or “cannot help”. However, after “cannot but,” the bare infinitive (base form of the verb) is used, and after “cannot help,” the gerund (verb + ing) is used.
Example:

• Affirmative: She must study hard to pass the exam.
• Negative: She cannot but study hard to pass the exam. (bare infinitive)
• Negative: She cannot help studying hard to pass the exam. (gerund)

must finish this work. Make it negative.
A. I must not finish this work.
B. I should not finish this work.
C. I cannot but finish this work.
D. I do not finish this work.

If you don’t comment we think you can not understand.