In some other Mahjong variants, you can win simply by getting 4 sets/sequences + 1 pair. However, in Japanese ("Riichi") Mahjong, not only you have to get 4 sets/sequences + 1 pair, but you also have to satisfy at least one of “yaku” conditions.
If you can prepare you hand for winning with 4 sets/sequences + 1 pair just by collecting tiles, without stealing a single tile from other players, then you will be able to declare “riichi”. Winning with declared “riichi” already gives you a “yaku”, so in this case there is no problem. You can pick the winning tile by yourself or get it from another player.
But if you start stealing other players’ tiles then “riichi” will not work. You will need some other “yaku”. The most practically useful “yaku” are following:
- a set of “dragons” (中 or 發 or the blank tile). This will always work.
- a set of “winds” (E東 or S南 or W西 or N北). This will only work if it is your “seat wind” or if it is the current “round wind”, otherwise it will not. If it is a computer game, it should display E or S somewhere, most likely in the middle of the table, which would be the “round wind”. The "round wind" is the same for all the players. Also, it should display E, S, W, N as the “seat winds” of each player. Each player has a different "seat wind".
- “tanyao”: no dragons, no winds, no “1”, no “9”. All the tiles are between “2” and “8” only.
- “honitsu”: only one suit + dragons and/or winds. For example, a set of South and everything else is bamboos. If you manage to get everything one suit without dragons/winds at all (for example, everything is bamboos only), this will be “chinitsu”, which is even better (will give you a lot of points).
You may wish to keep in mind also those, but they happen less often:
- “chitoitsu”: instead of 4 sets/sequences + 1 pair, your hand is 7 different pairs; you will obviously have to pick them up by yourself
- “toitoi”: no sequences at all; the hand is 4 sets + 1 pair
- “chanta”: every set/sequence/pair in your hand has a dragon or a wind or a “1”or a “9” in it.
There are also other "yaku", but they will only become useful for you later, when you start thinking about how to win more points, not just how to win.
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