替【tì】

替 + person before the verb says the subject does something that person would otherwise do themselves – instead of them, or on their behalf.

替 or 帮

帮 is help: you might do it with them or for them. 替 says you take their place.

For small favours the line blurs: 帮我买 and 替我买 can both mean "buy it for me".

Feeling something for someone

With 高兴, 担心 or 着急, 替 + person means "for" in the sense of "on their account". The feeling is yours; the reason is them.

English puts "for you" at the end. 替 + person goes in front of the adjective or verb.

• × 我很高兴替你 → 我真替你高兴