When joining the 接続助詞（せつぞくじょし）
て, with the
る-Verbあげる (to give), it implies that someone is doing something for the benefit of someone else. Literally, this expression means ‘to give/bestow the action of (A) upon (B)’, but translates more naturally as ‘to do (A) for (B)’, or ‘to do (A) as a favor for (B)’.
The receiver of the
てあげる action will be marked with
に, while the doer will be marked with
が (if mentioned).
-
父（ちち）が妹（いもうと）にお菓子（かし）を買（か）ってあげる。
My father buys snacks for my sister.
-
いいよ、俺（おれ）がやってあげるよ。
It's okay, I'll do it for you.
-
明日（あした）は私（わたし）が送（おく）ってあげるよ。
Tomorrow, I will send you off.
Unfortunately,
てあげる can sound quite patronizing in modern Japanese, and should be avoided in situations where someone may take offence to having something done for them (for example, doing something for someone of higher status). This is primarily due to
やる (a casual variation of ‘to give’) being phased out.
-
部長（ぶちょう）、今夜（こんや）は俺（おれ）がおごってあげるよ。
Boss, I'll pay for the food tonight. (Natural Japanese, but may be considered rude by the boss)
Originally,
やる was used when referring to giving water to plants, or feeding animals. However, some people started using
あげる for these purposes as well, believing that it sounded ‘nicer’. Historically, this actually achieved the opposite result, and some people began to think that
あげる was insulting, as it was being used for plants/animals.
These days,
てやる is almost never used, unless you have a very close relationship with someone and are using it jokingly.