A noun appearing as N. doer “bridegroom” in The Etymologies of the 1930s derived from the root ᴹ√NDER of similar meaning (Ety/NDER). It is unusual in that its primitive form is ᴹ✶ndǣr, a rare example of the a-fortification of primitive e to ǣ (PE18/46). In Old Noldorin this became ON. ndair, and in Noldorin of the 1930s the diphthong [[n|[ai] became [oe] or [ae]]]. Indeed, in another entry in The Etymologies, Tolkien gave a variant form daer for “bridegroom”, though somewhat mysteriously he marked it as Old Noldorin (Ety/DER).
Neo-Sindarin: The a-fortification of primitive e remained a feature in Tolkien’s later writings, though in the 1950s Tolkien marked the result as ę̄ rather than ǣ (PE18/95). Thus primitive √NDER > ✱ndę̄r > OS. ndair > S. daer remains a plausible scenario in Sindarin, but ai > oe no longer occurred as it did in Noldorin. Therefore, I’d use the form ᴺS. daer for “bridegroom” in Neo-Sindarin, as suggested in HSD (HSD).
Noldorin equivalent of ᴹQ. Ender, another name for ᴹQ. Tulkas (Ety/TULUK, EtyAC/E).