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?