Tap a free tile to send it to the staging area. When four tiles of the same category are gathered in a box, that box clears automatically and frees the space up again.

Almost always because staging filled up with several partly-collected categories at once. Committing to finishing one category before starting another keeps slots free and is the single biggest improvement most players can make.

Undo reverses your last tap, Shuffle rearranges the remaining tiles, and Hint suggests a useful move. You can also add extra boxes to give yourself more staging room when a board gets tight.

No. The categories are visual, so you are grouping objects that belong together rather than reading words. That makes it playable regardless of language.