Jesus uses a question to help the crowd understand that people are doing bad things now in good times, so certainly they will do worse things in the bad times in the future. Alternate translation: "You can see that they are doing these bad things while the tree is green, so you can be sure that they will do worse things when the tree is dry." (See: [[rc://en/ta/man/jit/figs-rquestion]])
The green tree is a metaphor for something that is good. If your language has a similar metaphor, you should use it here. (See: [[rc://en/ta/man/jit/figs-metaphor]])