xìn shì péngyou xiě de.

Yes, 信 has more than one meaning in Chinese.

In this sentence, 信 means letter.

So:

• 信 = letter
• 写信 = to write a letter

In other contexts, 信 can also mean things like:

• 相信 = to believe
• 信用 = credit / trustworthiness
• 信任 = to trust

Here, because it appears with 写 and the whole sentence is about something being written, 信 clearly means letter.

是...的 is a very common pattern used to emphasize some detail about a completed action.

In 信是朋友写的, the action is writing the letter, and the sentence highlights who wrote it.

So the structure is roughly:

• 信 = the letter
• 是 = is/was
• 朋友写的 = written by a friend / my friend wrote it

This pattern often answers questions like:

• Who wrote it?
• When was it done?
• Where was it done?
• How was it done?

Here, the emphasized detail is the person: 朋友.

A natural way to understand it is:

• This letter was written by a friend.
• It was a friend who wrote the letter.

In the 是...的 pattern, the final 的 is part of the grammar structure.

It does not simply mean possession here.

In this sentence, 的 helps turn 朋友写 into a phrase describing the completed event:

• 朋友写的 = the one that a friend wrote / written by a friend

So the sentence is not just a random 的 added at the end. It belongs to the pattern:

• A 是 B + verb + 的

Here:

• 信 = A
• 朋友写 = B + verb
• 的 completes the structure

Not necessarily.

When Chinese uses 是...的, the sentence already tends to imply that the action is completed, and the speaker is focusing on a detail of that action.

So:

• 信是朋友写的 = focuses on who wrote it
• 朋友写了这封信 = simply states that a friend wrote this letter

Because of that, 了 is often not used in 是...的 sentences.

This is a very common learner question, because English often marks past time clearly, but Chinese often chooses between different structures depending on what is being emphasized, not just on tense.

Because 朋友 here is the doer of the action, not a possessive phrase.

Compare these:

• 朋友写的信 = a letter written by a friend
• 朋友的信 = a friend’s letter

These are different ideas.

In your sentence:

• 朋友写的 means written by a friend

If you said 朋友的信, that would mean:

• the friend’s letter
• possibly a letter that belongs to the friend
• or a letter sent by the friend, depending on context

So 朋友 without 的 is correct here because it is the subject of 写.

It depends on context.

Chinese does not use articles like a, an, and the the way English does, so 朋友 can mean:

• a friend
• my friend
• the friend

The surrounding situation tells you which one is meant.

So 信是朋友写的 could mean:

• The letter was written by a friend.
• The letter was written by my friend.

If the speaker wants to be more specific, they can say:

• 我的朋友 = my friend
• 一个朋友 = a friend
• 那个朋友 = that friend

It certainly can.

A fuller version would be:

• 这封信是朋友写的。

That means:

• This letter was written by a friend / my friend.

Chinese often drops words that are obvious from context. If everyone already knows which letter is being discussed, just saying 信 is fine.

Also, 封 is the measure word for letters:

• 一封信 = one letter
• 这封信 = this letter
• 那封信 = that letter

So the sentence without 这封 is not wrong; it is just less explicit.

It has a passive-like meaning in English, but Chinese does not need 被 here.

English often says:

• The letter was written by a friend.

Chinese can express the same idea with 是...的:

• 信是朋友写的。

This is perfectly natural and often sounds better than using 被.

If you used 被, it would be:

• 信是被朋友写的。

That is grammatical in some contexts, but it usually sounds less natural here unless you really want to emphasize the passive relationship. 被 is more often used when the event has some special impact, contrast, or unwanted result.

So for a simple statement like this, 信是朋友写的 is the normal choice.

The sentence is:

• 信 / 是 / 朋友 / 写 / 的

A helpful way to read it is:

• 信 = the letter
• 是 = is/was
• 朋友写的 = the thing that was written by a friend

So the overall structure is:

• [thing] + 是 + [person/place/time/etc.] + [verb] + 的

Here the emphasized part is 朋友.

You can think of it as:

• As for the letter, it was a friend who wrote it.

This is one reason it may feel different from English word order.

These are related, but they are not the same.

1. 朋友写了信。

   • A friend wrote a letter.
   • This simply tells you an event happened.

2. 朋友写的信。

   • the letter that a friend wrote
   • This is a noun phrase, not a full sentence by itself.

3. 信是朋友写的。

   • The letter was written by a friend.
   • This is a full sentence using 是...的 to emphasize who wrote it.

So:

• 写了 = states the action
• 写的 before a noun = modifies the noun
• 是...写的 = emphasizes a detail about the completed action

Yes. That is exactly how 是...的 sentences are often used.

For example:

• 这封信是谁写的？ = Who wrote this letter?
• 信是朋友写的。 = The letter was written by a friend.

So this sentence works very naturally as an answer when the missing information is who did the writing.

Other 是...的 questions might focus on different details:

• 这封信是什么时候写的？ = When was this letter written?
• 这封信是在哪儿写的？ = Where was this letter written?

A few pronunciation notes:

• 信 = xìn fourth tone
• 是 = shì fourth tone
• 朋友 = péngyou
  • péng is second tone
  • you is usually neutral tone in normal speech
• 写 = xiě third tone
• 的 = usually neutral tone here, pronounced de

So in natural speech, the rhythm is roughly:

• xìn shì péngyou xiě de

Learners often try to give 的 a strong tone, but in this sentence it is usually light and unstressed.

Yes, absolutely.

That would mean:

• The letter was written by my friend.

Adding 我的 makes the meaning more specific.

Some other possibilities:

• 信是一个朋友写的。 = The letter was written by a friend.
• 信是我朋友写的。 = The letter was written by my friend.
• 信是那个朋友写的。 = The letter was written by that friend.

So the original sentence is somewhat general unless the context already tells you which friend is meant.