〜てあげる

do for someone

Says someone does something as a favour for someone else — usually the speaker, or someone on the speaker's side, doing it for others.

Readings

How to form it

Put the verb in the て-form and add あげる: 教(おし)えてあげる.
The person who benefits is often marked with に, but some verbs keep their own pattern: 友達(ともだち)に教(おし)えてあげた (I taught my friend), 友達(ともだち)の荷物(にもつ)を持(も)ってあげた (I carried my friend's bags).
あげる is an ichidan verb: あげます, あげた.

友達(ともだち)に日本語(にほんご)を教(おし)えてあげました。

I taught my friend some Japanese.

荷物(にもつ)を持(も)ってあげましょうか。

Shall I carry your bags?

Saying you do a favour for someone of higher status can sound condescending. Offer with humble お〜する instead: お持(も)ちしましょうか.