N4
てあげる
To do something for someone, As a favor for
父が妹にお菓子を買ってあげる。
My father buys snacks for my sister.
Explanation
「〜てあげる」 means 'do something for someone'. You (or your side) do something that benefits another person: 「友達に本を貸してあげた」 (I lent a book to my friend). Because 「あげる」 feels like granting a favour, you cannot say it straight to an elder or a superior.
Mixed up with
To an elder, do not say 「車で送ってあげます」; it sounds condescending. Say 「車でお送りしましょうか」.
The three differ in viewpoint: 「〜てあげる」 is from the giver's side; 「〜てくれる」 is from the receiver's side (someone does it for me); 「〜てもらう」 is that I ask for it or receive it.