半途而废

You usually use it as a verb or an adjective. You can say someone 半途而废ed. Or you can tell someone not to do it. It often follows words like 不要 (don't) or 总是 (always). It sounds a bit critical but very honest. Use it when the goal was actually reachable.

Use it when your friend quits gym after two days. Use it in a meeting about a dropped project. It works well for hobbies, diets, or big life goals. If you are texting a teammate who is giving up, use this. It adds a layer of 'hey, keep going' to the conversation.

Do not use it for things out of your control. If a storm stops a race, that is not 半途而废. That is just bad luck. Also, avoid it if someone has a valid, serious reason to stop. Using it for a medical emergency would make you sound like a jerk. Keep it for matters of willpower.

This idiom comes from a story about a scholar's wife. She cut a piece of silk she was weaving. She did this to show her husband something important. If you stop studying, your knowledge becomes useless like cut silk. Chinese culture deeply values the 'spirit of the constant' or 坚持. Finishing what you start is a major mark of character.

You might hear 功亏一篑. That is much more dramatic. It means failing at the very last tiny step. 半途而废 is more general for any point in the middle. If you want to be positive, use 坚持到底. That means to persist until the very end. They are two sides of the same coin.