5. Diphthongs are combinations of two vowels uttered as a single
sound, by one breath-impulse. The diphthongs in Esperanto contain an
i or u sound as the second element, but in order to avoid confusion
with combinations of vowels not forming diphthongs (as in "naiva", like
English "naive", etc.), they are written with "j" and "ux" instead.
Their pronunciation is as follows:
"aj" like "ai" in "aisle".
"ej" like "ei" in "vein", "ey" in "they".
"oj" like "oi" in "coin", "oy" in "boy".
"uj" like "ui" in "ruin", "u(e)y" in "gluey".
"eux" like "ayw" in "wayward", or like E(h)oo" pronounced together.
"aux" like "ou" in "out", "ow" in "owl".