白费力气...

You can use it as a standalone complaint. You can also use it to warn someone. It often follows a long description of a failed task. For example, 'I ran for the bus, but it left.' That is 白费力气. It usually sits at the end of a sentence to provide a punchy conclusion. You can also add 真是 (really) in front to add more emotion.

Use it when a plan falls through despite your hard work. It fits perfectly when technology fails you. Did you type a long email and the computer crashed? 白费力气. It is great for social situations too. Maybe you tried to convince a stubborn friend to try new food. If they refuse, you've wasted your breath and your effort. Use it to vent to coworkers about a canceled project.

Don't use it if someone actually succeeded but it was just difficult. That is just hard work, not 'wasted' work. Avoid using it to describe someone's sincere but failed romantic gesture to their face. It might sound a bit too harsh or dismissive of their feelings. Also, don't use it in very high-level diplomatic speeches. It is a bit too salty and informal for a state dinner.

Chinese culture highly values 'hard work' (努力). Because effort is so respected, the idea of it being 'wasted' is particularly painful. There is an old saying, 'Grinding an iron pestle into a needle.' That is the opposite of this phrase. 白费力气 represents the modern struggle of 'busy-ness' without 'business'. It reflects a pragmatic view of life: if there is no result, the effort is 'white' (empty).

You might hear 白忙活 (bái máng huó) in Northern China. This version feels a bit more 'earthy' and colloquial. Another one is 徒劳无功 (tú láo wú gōng). This is the fancy, four-character idiom version. Use that one if you want to sound like a scholar. If you are just complaining to your mom, stick with 白费力气.