てあげるDo something for someone
てあげる presents the subject's action as something done for another person's benefit. It views the event from the side of the person doing the favor.
Who does what for whom?
The subject is the person who performs the action. The person who benefits is often marked with に. Think of the pattern as describing an action that moves outward from the doer as a favor.
A clear giver-side example
父は祖母に新しいスマホを設定してあげた。
My father set up a new smartphone for my grandmother.
AnalyzeThe father is the doer, and the grandmother is the person who benefits.
The action must count as a favor
てあげる does more than say that an action affects someone. It presents the action as helpful or welcome to that person. Do not add it automatically whenever an English sentence contains “for someone.”
Offering welcome help
この箱、重くて運べない。
This box is too heavy for me to carry.
Analyzeじゃあ、私が入り口まで持ってあげるよ。
Then I'll carry it to the entrance for you.
AnalyzeThe offer responds to an obvious need, so presenting the action as help feels natural.
Giver-side versus speaker-side viewpoint
てあげる describes the action from the doer's side and points toward another beneficiary. By contrast, てくれる presents the action as benefiting the speaker or someone close to the speaker. In the pair below, the first sentence is an outside description; the second is what Akira could say about the same event.
More examples
姉は弟に面接の練習をしてあげた。
My older sister practiced for an interview with my younger brother to help him.
Analyze明日は私が子どもたちに絵本を読んであげます。
Tomorrow I'll read a picture book to the children.
Analyze道に迷っていた旅行者に駅までの行き方を教えてあげた。
I showed a lost traveler how to get to the station.
Analyze看護師は子どもに絵本を読んであげた。
The nurse read a picture book to the child.
Analyze母が忙しかったので、夕食を作ってあげた。
My mother was busy, so I made dinner for her.
Analyzeその荷物なら、車で家まで運んであげるよ。
If it's that luggage, I'll drive it to your house for you.
Analyze