为(wèi) - for, because of; to act as in Mandarin

为 (wèi) means for, because of, or on behalf of. 我为你骄傲 (wǒ wèi nǐ jiāo ào) is "I am proud of you", and 为人民服务 (wèi rén mín fú wù), "serve the people", is the slogan still painted on walls across China.

The trap is the tone, and it is worth meeting early. Read wèi, fourth tone, 为 points at the reason or the person you are acting for: 为什么 (wèi shén me), "why", is literally "for what". Read wéi, second tone, the same character means to do, to act as, or to be: 成为 (chéng wéi) is to become, 认为 (rèn wéi) is to reckon, and 以为 (yǐ wéi) is to think something that turns out to be wrong. Nothing in the writing separates the two, so learn the compound rather than the character on its own.

One more distinction that catches beginners: 为 and 为了 (wèi le) are not interchangeable. 为 usually takes the person or cause you are acting for, so 为你 (wèi nǐ) is "for you". 为了 takes the purpose you are acting towards, so 为了钱 (wèi le qián) is "for the sake of money".