てあげる【～てあげる】

てあげる means doing something for someone else: the て-form of a verb plus あげる, "give". The one doing the favour is you, or someone on your side, and the one receiving it is outside your circle.

The particles stay the verb's own. The person takes に only when the verb itself takes に, as 読む, 貸す and 教える do. Otherwise the person goes where the verb puts them: with の on the thing, or with を.

• × おばあさんに荷物を持ってあげました → おばあさんの荷物を持ってあげました

てあげる, てくれる and てもらう

The three describe the same favour from different sides. てあげる goes from you outward. てくれる comes in towards you: someone does it for you. てもらう has you as the subject, getting it done, with the doer marked by に.

• × 田中さんが私に荷物を持ってあげた → 田中さんが荷物を持ってくれた

Saying it to the person

To someone's face, てあげる points at your own kindness. Among friends and family that is fine, but to a superior or a stranger it sounds condescending. Offer with the humble お〜します instead.

• × 先生、荷物を持ってあげます → 先生、荷物をお持ちします

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てさしあげる is the more polite form, used when talking about a favour done for someone of higher status. It still announces the favour, so お〜する remains the safer choice to their face. てやる is the casual form, for favours to animals, plants and your own children, and it sounds rough in other uses.