にとって: whose perspective makes something important?
Identify whose perspective supports an evaluation such as importance or difficulty. Distinguish this from the recipient of an action or benefit, and read evaluations from different viewpoints.
The same thing can receive different evaluations
私にとって、この写真は大切です。 means that this photograph is precious to the speaker. It identifies whose perspective gives it value rather than stating its objective price.
この説明は初心者にとって難しいです。 evaluates the explanation from a beginner's perspective. It does not claim that everyone finds it difficult; an experienced person might evaluate it differently.
Distinguishing the target of an action
学生に対して説明しました。 describes explaining something to students. 学生にとって分かりやすい説明でした。 evaluates the explanation as easy for students to understand, so にとって plays a different role from に対して.
友達にとって手紙を書きました does not naturally express writing a letter to a friend. Mark the recipient with に in 友達に手紙を書きました; whether that letter matters to the friend is a separate evaluation.
Contrasting viewpoints and modifying nouns
私にとっては簡単でも、弟にとっては難しい問題です。 describes a problem that is easy for the speaker but difficult for their younger brother. は contrasts the two evaluators without implying that either evaluation is false.
子どもにとっての遊びの大切さを考えます。 uses の to form a noun phrase about the importance of play for children. Unlike 子どもの遊び, it marks the perspective of evaluation, not simply whose play is involved.
Quick check: Does 私にとって、この仕事はやりがいがあります。 mean that the speaker assigned work to someone?
Show the answer and why
No. It evaluates this work as rewarding from the speaker's perspective. にとって marks that perspective, not the recipient of an action.