Examples of errors:
Must is used in senses parallel to should (in sense 2--cf. above), but more emphatically-.or categorically; i.e. where should or ought to indicates desirability and moderate probability, must expresses necessity or obligation and high probability. (l)-(5)-describe situations that do not exist but are desirable, so should (sense 2) or ought to is the most appropriate form. It is true that the desirability of a state of affairs can sometimes be expressed emphatically or rhetorically by expressing it as a necessity or obligation, in which must is conceivable (e.g. Unemployment must be eliminated!). This rhetorical tone is not appropriate in the contexts in which (l)-(5) occur, however. Furthermore, as Leech (1971:§115) points out, must in this sense normally requires that the speaker is the person in authority, the one who is imposing the obligation. For example, a sentence like Unemployment must be eliminated! would be assumed to have been uttered by a politician, or member of a political party, who would conceivably be (or hope to be) in a position to exert some influence on the situation. This implication of the speaker's authority is not appropriate in (l)-(5).
A particularly important restriction on must for German speakers is that must not is not equivalent to müUssen nicht. Müssen nicht expresses the lack of obligation; must not expresses negative obligation--the obligation not to do something:
lack of obligation
Du mußt nicht hingehen.
You don't have to go.
You needn't go
negative obligation
Du darfst nicht hingehen.
You musn't go.
You can't go.
In (6)-(9) lack of obligation is the intended meaning, for which don't have to or needn't/don't need to is the correct expression.