Why is 'cum' followed by the dative in this sentence?

Question

Look at the following sentence from Orberg's Lingua Latina Per Se Illustrata - Familia Romana (ch. 18).

Consonans per se syllabam non facit, sed semper cum vocali in eadem syllaba iungitur.

The word vocalis, vocalis (f) is written in the dative singular here instead of its ablative singular vocale. Why is that? Can cum be followed by the dative in certain cases?

Answer 1

It's actually the ablative, not the dative. It's an i-stem, and Latin allows some i-stems to have an ablative singular in ī. I've copied the relevant section from Allen and Greenough below:

76. The regular form of the ablative singular of i-stems would be -ī.

sitis, sitī

But, in most nouns this is changed to -e.

a. The Ablative in -ī is found exclusively—

1. In nouns having the Accusative in -im (§ 75 above); also secūris.
2. In the following adjectives used as nouns. 

aequālis, annālis, aquālis, cōnsulāris, gentīlis, molāris, prīmipīlāris, tribūlis

3. In neuters in -e, -al, -ar except: baccar, iubar, rēte, and sometimes mare.

b. The Ablative in -ī is found sometimes—

1. In avis, clāvis, febris, fīnis, īgnis,1 imber, lūx, nāvis, ovis, pelvis, puppis, sēmentis, strigilis, turris, and occasionally in other words.
2. In the following adjectives used as nouns.

affīnis, bipennis, canālis, familiāris, nātālis, rīvālis, sapiēns, tridēns, trirēmis, vōcālis

Do look over the whole page, as there are some exceptions.