A correct True or False answer raises a word's mastery by half a level rather than a full one, so it takes two of them to move a word up a step. The reason is arithmetic: with two options you are right half the time by guessing, and a schedule that treats a coin flip as proof of recall will schedule words you cannot actually remember.
The weight, and where the other half goes
Each mode passes a weight between 0 and 1 when it records a correct answer. Typing a word from memory passes the full amount. True or False passes half. That half is not discarded — it accumulates as partial credit on the word, and when the partial credit reaches a whole, the mastery level goes up.
Storing the fraction matters more than it sounds. If half-answers were simply rounded away, whether your second correct answer counted would depend on whether you happened to have an odd or even number of previous answers. Keeping a running total means two halves reliably make one, whenever they arrive and in whatever order.
Wrong answers are never discounted
The weight applies to correct answers only. Get a True or False question wrong and the word drops a full mastery level and becomes due immediately, exactly as it would in any other mode.
That asymmetry is intentional and it is the right way round. A lucky guess is weak evidence that you know something. A wrong answer is strong evidence that you do not, whatever format it arrived in — there is no version of picking the wrong option out of two that leaves your knowledge in doubt.
So is True or False worth playing?
Yes, for what it is good at. It is fast, it is the easiest mode to play in a queue or on a phone with one hand, and it is a low-pressure way to meet unfamiliar words for the first time. It also earns less XP per correct answer than most other modes, which keeps the incentives pointing the same way as the measurement.
What it cannot do is carry a word to mastery on its own at the speed the other modes can. If a word matters to you, meet it in a mode that makes you produce it.