All 2-letter words
This is the highest-value list in any word game, and it is short enough to actually learn. Two-letter words are how you play parallel to an existing word instead of perpendicular to it — laying a tile alongside a word on the board so it forms two or three short words at once, each scoring the full bonus square underneath. A player who knows this list scores on turns where a player who does not simply passes.
The ones people get wrong
Most of the surprises are in the vowel-heavy corner. Aa is a kind of lava, ae and oe are Scottish and Faroese respectively, and ai is a three-toed sloth — all four are accepted almost everywhere and all four look like typing errors. In the other direction, plenty of things that feel like words are not: there is no ja, no ok, no po, and no za in this dictionary, though some game-specific lists add the last two.
The ones worth points
Only a handful contain a high-value tile: ax, ex, ox, xi and xu for the X, and jo for the J. That makes them disproportionately useful — ox played across a triple letter square scores more than most four-letter words, and jo is very often the fastest way to unload a J you have been carrying for three turns.
Take the two-letter word quiz — twenty pairs, half of them real. It is a faster way to find the gaps than reading the list again.
The complete list
Have a rack in front of you? Put the letters into the unscrambler and set the maximum length to two to see only what your own tiles allow.