～てやる

～てやる adds a rough, masculine nuance to the verb phrase, indicating the speaker will take action for someone else, usually with a sense of determination, practical help, or a challenge to the listener. It is often used when the speaker wants to express that they will do something for another person, especially something beyond what is expected, or to emphasize that they will take action to prove something. It can also carry a slightly brash or casual tone, so it is more common in informal speech and among men, friends, or younger people. In some contexts it can convey a sense of “I’ll show you” or “you’ll see” as a consequence of the action. Typical usage includes offering help, completing a task for someone, or making a bold promise to prove something. Compare with てあげる, which is more neutral or polite, whereas てやる has a tougher, more assertive nuance.