I noticed something in Orberg’s LLPSI Ch. XIV.
‘Alter e duobus pueris valet, alter aegrotat.’
It maybe that this is explained but I either didn’t see it or can’t remember it. I didn’t notice the use of the ablative. Can anyone explain…? I would have expected Alter puer valet, alter aegrotat. I don’t understand the construction…
Alter ē/ex duōbus puerīs = one of the two boys
The ablative is because ē/ex always takes the ablative. (For future reference, you will also see dē used this way, also with the ablative. There is no difference in nuance or meaning that I know of.)
I would have expected Alter puer valet, alter aegrotat.
This is valid too, but the grammatical role of ‘alter’ and ‘puer’ differs between the two. In your sentence, ‘alter’ is an adjective modifying the subject, ‘puer’. In Orberg’s sentence, ‘alter’ is (acting as) a noun, and puerīs is the object of ē.
See the difference? 
Thanks. It’s just that I wasn’t aware that the ablative could be used in that way to make a simple statement like this. I can’t recall any previous examples in LLPSI - unless I have amnesia..and Orberg doesn’t draw attention to it or explain it. Again I may have simply forgotten..I’ve been so focused on dative of interest etc…