てくれる is used when someone does something for you (the speaker) or for someone in your in-group (like family). It highlights the benefit you receive and often carries a sense of gratitude or appreciation. The subject is the person doing the favor, and the receiver is usually 'me' or 'us'.

In conversation, てくれる is common and natural. The past form てくれた is often used to talk about favors already done, and てくれます is the polite form. You can also make a request with てくれませんか, which is a polite way to ask someone to do something for you.

Be careful: てくれる always implies the action is done for the speaker's benefit. If the action is for someone else, you might use てあげる instead, but てくれる is only for the speaker or their group.