Quick answer
The safest discard is a tile already thrown and not called, because anyone who needed it would have taken it, though a tile that was safe earlier can still complete a hand later. The most dangerous are fresh tiles nobody has thrown, especially in a suit drying up in the discards, and winds and dragons late in a hand. Read each opponent's exposures for the card lines they point to, and turn to safe discards when you are still four or more tiles away about two thirds of the way through the wall.
Half of American Mahjong is building your hand. The other half, the half beginners never get taught, is not handing somebody else theirs. Since the player who throws the winning tile pays double, defense is worth real money, and it starts the moment the first set goes face up.
Exposures are a confession
When an opponent exposes three of a number, they have told you a great deal. Combined with the card, three of anything narrows their possible hands to a short list, and every line on that list needs particular tiles. You do not need to identify their exact hand, only the neighborhood.
Watch the suits in their exposures too. Two exposures in the same suit could point to a single-suit hand, so the tiles near their numbers in that suit deserve care. Exposures in two different suits rule a single-suit hand out.
The safest tile is one already discarded
A tile that has been thrown and not called is close to safe, because if someone needed it they would have taken it. Late in a hand, matching a previous discard is a sound defensive habit, though a tile that was safe earlier can still complete someone's hand now.
Next safest is a tile whose other three copies you can already see, between the discard pile and exposures. Nobody can want it for a pair, because a pair needs two real tiles and a joker can never be part of one. It can still be called for a set built with jokers, or as the tile that finishes a hand, so it is safer, not safe.
The dangerous ones
Fresh tiles nobody has thrown, especially in a suit that is drying up in the discards, are the most likely to complete somebody's hand. So are winds and dragons late in a hand, since those sit in a lot of card lines and get held to the end.
The tiles that fit a visible exposure are the dangerous ones, like the next number up or down in the same suit as an exposed pung, which a consecutive-numbers line would call. A tile that matches an exposed set is a different case. An exposure is fixed once its owner has discarded, so if someone has three 5 Bams exposed, throwing the fourth cannot add it to their pung.
When to switch to defense
Around two thirds of the way through the wall, take an honest look. If you are still four or more tiles away with no realistic draw path, your hand is not going to land. That is not failure, that is information.
From then on the goal changes from winning to not paying double. Discard safe tiles, break up your own hand if you have to, and let someone else take a small pot rather than handing them a large one.
Defense without paranoia
You can overdo this. A player who only throws safe tiles never builds anything and loses slowly instead of quickly. Early in a hand, build first, because nobody is close enough for your discards to matter.
The switch is what matters: aggressive early, watchful in the middle, careful at the end. Knowing where you are on that curve is most of the skill.
Put it into practice
New to the game? Start the course at Lesson 1, Meet the Tiles. The first three lessons are free and need no account.
Already play? Have the coach read a real dealt hand for you, free. It names the direction the hand goes for, what to keep, what to pass, and the reasoning behind each call.
Defense starts with knowing whether your own hand is still alive. Our AI coach reads a dealt hand, tells you how strong it actually is, and explains what it would take to get there, free.