Actually, as is often the case, both your examples are acceptable.
You are correct in recognizing that there is a "rule" in
reported speech, as in your example: the verbs coming after a past tense
reporting verb—told in this case—often
appear in a past tense form.
One correct way to state the sentence you cite would be:
(a) |
He told them again that they had to
learn to love each other.
|
However, the alternative way to express this kind of sentence keeps
the modal must in its present form:
(b) |
He told them again that they must learn
to love each other.
|
In fact, Quirk (A Comprehensive Grammar of the English Language,
Longman, 1985) states:
| In its obligational sense... the past of must
may be replaced (italics added) by had to in indirect speech: |
|
His parents told him that he must/had
to be in by ten that night.
|
So, this reliable reference tells us that both must and had to are
properly used in this kind of sentence. In addition, we see authentic
examples of both in current use in the Collins COBUILD online (https://titania.cobuild.collins.co.uk/cgi-bin/democonc).
Of the first 40 examples in each of the following searches, we found:
1) the reporting verb “said,” appears with “had to”
in the dependent clause following it 25 times, as in these examples:
The survey said actors, models and musicians had
to…
…Nick Partridge said that more had
to be done to reach people…
2) the reporting verb “said” appears with the “must”
of obligation following it 11 times, as in these examples:
Lord Mackay’s letter said that judges must
act and must be allowed to continue…
We said that these two assumptions must
be tested before any of the…
3) the reporting verb “told” appears with “had to”
in the dependent clauses following it 30 times, as in these examples:
…teleconference panelists told callers that
they had to help improve the…
…damage the blood vessel walls, and told her
that she had to limit her intake…
4) the reporting verb “told” appears with the “must”
of obligation following it 19 times, as in these examples:
…at Arlington Cemetery. There he told Americans
that they must…
The letter told Morgan that he must
clear all his…
Had to appears almost twice as frequently as must
in these dependent clauses following the reporting verbs said
and told. This suggests that had to
is indeed more common than must in the dependent clause,
as in the example question. However, must seems to
be retained in more often when it represents a strong moral, ethical,
or legal obligation, more than it is for an “everyday” one.
(To
see a comment on this topic, click here.)
(To
see a related comment,
"Have
to vs. must," click here.)