Meaning of the negation in phrases like "не должен прочитать"

Question

I am often confused by phrases like "не должен прочитать," "не должен сделать," "не должен работать," "не должен платить" and so on. Sometimes the negation appears to refer to "должен," and sometimes - to the subsequent verb (i.e., to "прочитать" or whatever follows after "должен"). That is, I often get confused as to which interpretation is correct - "is not obliged to read" or "is obliged not to read."

My question is this: Is there any simple rule or trick to determine which verb is implied to be negated in phrases like "не должен прочитать"?

There might be some rule I am not aware of. Or should I always assume that both interpretations are possible unless one of them is excluded by the context?

Answer 1

Usually (usually), imperfective means "should not be doing it":

Он не должен это читать // He cannot be required to be reading this

and perfective means "should be not doing it":

Он не должен это прочитать // He is not allowed to read that.

Answer 2

I would say - no trick needed. "(не должен) прочитать" has the same meaning as "должен (не прочитать)" - there is no differnce in result: you have an obligation to that book not to be read. There is no significant difference.

But if you want a rule - in general "is obliged not to read" is prefered in almoust any case ( when I translate Russuan to English I have to watch context of English phrase).

"Not obliged to read" should be translated to Russian as "не обязан читать"