に to indicate the role you want something to play?

Question

In Chapter 11 of the Genki textbook, it's stated that に can be used to indicate the role you want something to play. The example sentence given was:

お土産に絵葉書を買いました。

However, I am a bit confused as I was unable to find this particular usage of the に particle elsewhere. Could someone please clarify this issue for me?

Also, I was under the impression that the で particle is used to give context to things. So is it possible to replace に with で in the example sentence above?

Answer 1

I was unable to find this particular usage of the に particle elsewhere.

Maybe you could try a monolingual dictionary...? For example, 明鏡国語辞典 says:

に 🈩〘格助〙
➊㋖資格や見立て、名目などを表す。…として。 [...] 「本を[枕]{まくら}にする」「お土産に菓子をもらう」「お礼にひとこと」

This に can usually be rephrased as として. It can usually be translated to "as~" or "for~".
本を枕にする -- use a book as a pillow
お土産に菓子をもらう -- given sweets as a souvenir/gift
お礼にひとこと -- a word as expression of gratitude -> say something to show one's gratitude
So.. as your textbook says, the に is used to indicate the role one wants something to play: one makes 本 play the role of 枕, 菓子 the role of 土産, and ひとこと the role of お礼.

Or maybe you could try プログレッシブ和英中辞典:

に
7 〔…として〕as; for
親切にしてもらったお礼 (の印) に彼らを招待した
He invited them 「as a token of his gratitude for [in return for] their kindness.
誕生祝いに時計をもらった
He was given a watch for his birthday.

I was under the impression that the で particle is used to give context to things. So is it possible to replace に with で in the example sentence above?

Grammatically speaking, I think 「お土産/プレゼント/お礼に～」 is the correct usage, and using に sounds more correct to me (of course..), but I occasionally hear/say things like 「お土産でXXをもらった」「これ、誕生日のプレゼントでもらった」 etc. in casual conversation... (I don't hear/say 「お礼でひとこと」「XXをYYのお礼で差し上げる」, though...) So.. I think it'd be probably okay to say 「お土産・プレゼントで～～あげた・もらった」 in casual speech but you should use ～～に in formal speech, writing, exams, etc. 　

Answer 2

に cannot be replaced by で in the above sentence.

It is really hard to pin down the exact translation of に because it has many, many usages. As a rule of thumb though, に emphasizes the goal / end point of an action. You could formulate it as purpose in your case. Another similar example would be:

• 父の遺産は家のリフォームに使いたいと思います。I want to use my father's heritage in order to renovate my house (goal)

On the other hand, で means "to use" and / or emphasizes the usage of something as a means to an end. For instance:

• ２時間で走りきった。I "used" two hours to run the entire distance
• お金で買いました。I used money to buy it.

As you can see from the last example, using で in お土産でハガキを買いました sounds like you used souvenirs in order to buy postcards...