Rule: The unexpressed subject of the infinitive following be to must also be the subject of be.
Examples of errors:
(1) It is to remember that the child's ability to learn his mother tongue is inborn.
(2) In general it is to say that the cafeteria should be enlarged, because there isn't enough room to sit down.
(3) In general it is to say that unborn life must be protected by law.
(4) It is to say that there was never a law against 'Lesbians but only against male homosexuals.
(5) The baby uses this experience and gradually habits are established. It is to say that these habits are largely phonetic.
(6) Why this is a healthy development is hardly to explain.
In sentences (l)-(5) the intended meaning of the be to + infinitive construction is probably that of 'obligation'. or perhaps that of 'possibility'; the meaning in (6) is 'possibility'. The problem in all of these sentences is that English does not allow a passive interpretation of the infinitive, as German does. Compare:
Es ist zu erinnern daß...
It is to remember that...
It is to be remembered that...
In other words the subject of be must also be the unexpressed subject of the infinitive. In It is to remember that..., it is not the (unexpressed) subject, but the object, of remember; in order for it to be construed as the subject of remember, the infinitive must be passive. Another way to express Rule 2.7.1 is to say that be to must be replaceable by a modal verb. Thus
*It must remember that...
is of course also incorrect since in the modal construction, too, the underlying subject of the infinitive must be the subject of the main verb:
It must be remembered that...