mise de binsen wo katte, tomodati ni tegami wo kakimasita.

で marks the place where an action happens.

• 店で = at the store / in the store
• The action of 買う happens there, so で is the correct particle.

Compare:

• 店で買いました = I bought it at the store
• 店に行きました = I went to the store

So に often marks a destination, while で marks the location of an action.

を marks the direct object of a verb — the thing directly affected by the action.

In this sentence:

• 便せんを買って = bought stationery / writing paper
• 手紙を書きました = wrote a letter

So both nouns are objects, but they belong to different verbs:

• 買う takes 便せん
• 書く takes 手紙

That is why を appears twice.

They are related, but not the same thing.

• 便せん（びんせん） = letter paper / writing paper
• 手紙（てがみ） = a letter

So the sentence says the speaker:

1. bought letter paper
2. then wrote a letter

In other words, 便せん is the paper/material, while 手紙 is the actual message or finished letter.

This is a very common pattern in Japanese.

The て-form can connect actions in sequence:

• 買って、書きました
• bought ... and then wrote ...

Usually, only the final verb shows the main tense and politeness clearly. So:

• 買って = connective form
• 書きました = final verb, polite past

Because the last verb is past polite, the whole sequence is understood as past polite.

Yes, most naturally it means:

1. bought the letter paper
2. wrote a letter to a friend

The て-form often links actions in the order they happened, especially in simple narrative sentences like this one.

So the sentence strongly suggests a sequence, not just two unrelated actions.

Here に marks the target or recipient of the letter.

• 友達に手紙を書く = to write a letter to a friend

This is similar to how に is often used for the person toward whom an action is directed.

Examples:

• 母に電話しました = I called my mother
• 先生に聞きました = I asked the teacher
• 友達に手紙を書きました = I wrote a letter to my friend

Usually, に is the normal choice with 手紙を書く when marking the recipient.

• 友達に手紙を書く = standard and natural

Using へ can sometimes appear in expressions involving direction, especially with sending or addressing something, but with 書く, に is the more basic and expected particle for the person you write to.

So for learners, 友達に手紙を書く is the pattern to remember.

Japanese often leaves out information that is obvious from context.

Once you say:

• 便せんを買って = bought letter paper
• 手紙を書きました = wrote a letter

it is easy to understand that the paper was used for the letter. Japanese usually does not repeat details unless they are important.

This is very natural. Japanese often sounds less repetitive than English in this way.

Yes. 友達に手紙 means a letter to a friend, not a friend’s letter.

Compare:

• 友達に手紙を書きました = I wrote a letter to a friend
• 友達の手紙 = my friend’s letter / a letter from a friend depending on context

So に shows the recipient, while の would show a noun relationship like possession or association.

The final verb is 書きました, which is the polite past form of 書く.

Japanese sentences often show politeness mainly through the final verb. Since the sentence ends with 書きました, the whole sentence is in polite style.

The first verb, 買って, does not itself show plain vs. polite in the same way because it is just connecting to the final verb.

A plain-style version would be:

• 店で便せんを買って、友達に手紙を書いた。

Yes, in natural English you might translate it that way, but in Japanese 手紙を書きました specifically says wrote a letter.

Japanese often uses the noun plus verb combination:

• 手紙を書く = write a letter
• 本を読む = read a book
• 写真を撮る = take a photo

So even if English sometimes says write to a friend, the Japanese here literally includes letter as the object.

店 just means store/shop, so it is a general word.

The sentence is grammatically complete with 店, even if it does not specify what kind of store it was. Japanese often leaves things general when the exact detail is not important.

If you wanted to be more specific, you could say:

• 文房具屋で = at the stationery store
• デパートで = at the department store

But 店で is perfectly natural if the specific type of shop does not matter.