Early Quenya
ar(a)
conjunction. but; and
ya(n)
conjunction. and
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
The usual word for “and” in the 1910s was ᴱQ. ya(n) (PE15/69, VT40/8). It was derived from the early root ᴱ√YA (or possibly ᴱ√(D)YṆTṆ) in the Qenya Lexicon of the 1910s (QL/104-105). In his later writings, the usual word for “and” was Q. ar, but something like ya(n) survived in the “dual” conjunction yo “both ... and”.