Translation:It is necessary that the baby receive enough milk.
Yes, it is the subjunctive, which is used in cases where you aren't speaking about facts, but desires, beliefs, intentions, etc, which is when you would use the indicative form in Esperanto. The only times where it's actually noticeable in English is with third person singular verbs where the -s is dropped, and also with the verb "to be", where in the present "be" is used for all subjects ("I wish that she be notified"), and in the past "were" is used ("I wish that she were notified").
Because it also functions as the subjunctive. In English you can (and should) say "it is necessary that the baby receive enough milk" rather than "receives", just like in Esperanto you have to use a different ending to express that you are talking about necessity.
It makes a lot of sense to use the imperative, since saying that something is necessary is basically the same as giving an indirect order to someone, and, in fact, it's exactly the same in English: "it is necessary that the baby receive enough milk" = "you, baby, receive enough milk!".
For those who may still be confused as to why the subjunctive shares the same tense as the imperative:
It may help to think of the imperative as a special case of the subjunctive. When you tell someone to do something, you're really expressing the DESIRE that they comply with your demands.
Even if that desire is backed by the full force of authority (e.g. parent to child, or boss to subordinate) it's still expressing the thought that, "I want you to do [X]." And wanting to have something happen is the very definition of the subjunctive.
[Many thanks, by the way, to the Spanish language for helping the above make sense to this native English speaker.]
I would not say that the subject is missing. Rather, I would say that the subject is the subordinate clause "ke la bebo ricevu sufiĉe da lakto" itself. And, what I cannot understand is why cannot I express a predicate from a subordinate clause? Ekz. ("ke vi venos") estas (bona ideo). Is that wrong? Since "ke vi venos estas bone ideo" is not the same thing. Thanks for the explanation
Not the best argument, as there are other things that can be done in English, that do not work in Esperanto. I have never come across a sentence that started with ke. (But that of course doesn't mean it isn't allowed.)
EDIT: I've seen sentences start with ke; this seems fine.
The sentence indicates a quantity of milk, not some part of milk or something that belongs to or otherwise relates to milk.
Botelo da lakto = bottle of milk → the amount of milk described by how much of it is in a bottle; the bottle is full of this amount of milk.
Botelo de lakto = milk bottle → bottle meant for milk, no indication as to whether or not there is actually milk in there.
So sufiĉe da lakto describes a quantity of milk, namely ‘sufficient’.
I think the reason is that sufiĉa would just relate directly to the thing it was describing: La bebo ricevu sufiĉan lakton. (which sounds kind of weird to me, but stitll). The expression with sufiĉe da is essentially equivalent to sufiĉo(n) da. So
La bebo ricevu sufiĉe da lakto.
is the same as
La bebo ricevu sufiĉon da lakto.
That's the only reason I can think of. Saying sufiĉa da is ungrammatical (line 12, in Esperanto).
More information about how this construction is used can be found here (in Esperanto).