Learn English – ought not to do or ought not do

To my ear, the PEU examples are both colloquially natural and accurately described, while the Macmillan examples, although colloquial, are somewhat misleadingly described: things that are happening now might be better expressed as things that have just happened or have just been reported. But that's a minor criticism.

All these examples exhibit the same basic use of will to express habitual or characteristic action. In this sense (like all the modals in all their senses) this habitual will employs its tensed forms, and particularly the past-tense form, to express nuances which may have little or nothing to do with tense. Speaking very broadly,

• Ordinary unstressed uses usually express simple habit—behavior repeated frequently over a long period of time—and tense has its usual significance of present or past reference:

  When he's preoccupied with a problem he will often pace up and down for hours.
  When I was a child we would always go to my grandmother's for Thanksgiving dinner.

• When will or would is stressed, the sense is somewhat different: the habitual action is represented as perversely deliberate. And the tenses are employed somwhat differently, too. The present-tense form generally signifies that the subject currently makes a habit of the perverse behavior:

  She will keep falling in love with the wrong people is rhetorically equivalent to She insists on falling in love with the wrong people.

  But the past-tense form, although refers to a past action, does not express repeated action in the past but represents the subject's behavior on a single occasion as characteristically perverse:

  You would tell Mary about the party ... Even though I asked you not to tell anybody without checking with me, you just can't control yourself around her.

  A: He said it was all Jack's fault.
  B: Well, he would say that, wouldn't he. Nothing is ever his fault, oh no!

The second one is correct:

You mustn't have done anything to fix it.

Not sure if there is a general rule, but at least with the construction "must have done", if you want to negate it, the "not" comes after the "must." The "negative" part could come after the "must have" part if you replace either "to do" or "anything" with a word that is basically the opposite, e.g.:

You must have done nothing to fix it.

You must have neglected to fix it.

These both have more or less the same meaning as the original sentence.