+178.+ _Must_ and _ought_ imply obligation. _Must_ conveys the idea of
being obliged to do an action from necessity or compulsion, as,
You must have known it.
He must go.
_Ought_ was originally the past time form of _owe_, hence means _to be
indebted to_, _to owe_. It conveys the idea of a moral obligation, as,
You ought to help the cause.
You ought to understand.
+179+. _Ought_ is always used with the infinitive, and the same form is
used to express both the present and the past time. The difference in
time is expressed by a change in the infinitive instead of a change in
the form of the helping verb. With _may_ and _might_ and _can_ and
_could_, present and past time are expressed by a change in the form of
the helping verb. With the helping verb _ought_, the difference in time
is expressed in the infinitive. For example:
He ought to pay us our wages.
This means, _He owes it to us to pay us our wages now_.
He ought to have paid us our wages.
This means, _He owed it to us to pay us our wages some time in the
past_.
+180.+ The present infinitive is used with the helping verb _ought_ to
express present time and the perfect infinitive is used with _ought_ to
express past time.