SPOILER
As far as I can understand through the end,
T(Truths): Can only speak truths.
D(Dare): Can do both, but lying would cause them pain.
F(False): Can only speak lies, if they're trying to speak truth - words won't came out from their mouth.
There's no scene where T do the act of writing and such, unlike when saegye (F) was doing some alternative way to speak the truth.
And just like what I've said, Dare people can actually speak both. Telling truth is normal for them, but the issue is that telling lies would cause them pain.
Also, the truth in the world in this story was the highest or most likely belived one so lying wasn't really on their mind, after all — they believe that their world only has the truths.
For the question about crime and such, their world there was really peace (in their perspective) because they believe that only truth exists. While kf course, some dares are able to do such crimes as they are able to lie.
Oh LMAO, I'm so sucked up with the story. Have you finished the manhwa? I didn't noticed the date of your topic and it was remained unanswered. (⊙…⊙ )
Question, can truths only not speak lies, or are they able to write them. If truths are mute, they would either have to sign everything or write everything, but are they capable of lying in writing or signing? Same with lies, and they write or sign truths? If they all can then if you want to commit a crime or do something wrong, all you have to do is pretended to not be able to speak right? I don’t know cause I’m only on chapter four so this might have been answered in a later chapter