Learn Hardcore Japanese: sensei ha watasi ni syoukaizyou wo kaite kurete, mensetu de sore wo misemasita. - The teacher wrote a letter of introduction for me, and I showed it at the interview.

先生は sets up “the teacher” as what we’re talking about for this sentence (or paragraph).
Using 先生が would put more emphasis on who did the action, like: “It was the teacher who wrote it,” which feels more contrastive or focused.

In this sentence, the main point is the situation (the teacher wrote me a recommendation and I showed it), not specifically contrasting the teacher with someone else. So 先生は is natural as a topic marker:
先生は (as for the teacher), 私に紹介状を書いてくれて、…

What is the role of に in 私に紹介状を書いて?

The に after 私 marks the indirect object or beneficiary of the action.

私に紹介状を書く = “to write a letter of recommendation for me / to me.”

Here, 私に shows that the action of writing the recommendation is directed toward you, or done for your benefit. In English this becomes “wrote a letter of recommendation for me.”

What does 書いてくれて mean exactly, and how is it different from just 書いて?

書いてくれて is 書いて (te-form of “to write”) + くれて (te-form of くれる).

書いて: just “wrote (and…)” – a neutral linking of actions.
書いてくれて: “wrote (it) for me (as a favor).”

くれる means “to give (to me / to us)” and, when attached to the te-form of a verb, it adds the nuance “someone did [verb] for my benefit.”

So:

先生は紹介状を書いて… = The teacher wrote a letter of recommendation (neutral).
先生は私に紹介状を書いてくれて… = The teacher kindly wrote a letter of recommendation for me (favor, helpfulness).

This くれて carries a subtle feeling of gratitude or benefitting the speaker.

Why is it 書いてくれて and not 書いてくれました?

Because 書いてくれて is being used to connect to the next action 面接でそれを見せました.

Japanese often links multiple actions by putting the earlier ones in te-form and making the last verb carry the main tense and politeness:

先生は私に紹介状を書いてくれて、面接でそれを見せました。
→ “[The teacher] wrote me a recommendation letter (for me), and (I) showed it at the interview.”

If you said:

先生は私に紹介状を書いてくれました。そして、面接でそれを見せました。

this is also correct, but it sounds a bit more “step‑by‑step,” like two separate sentences. The original sentence flows more as one connected event. The te-form 書いてくれて naturally links into 見せました.

What nuance does くれる have compared with もらう, as in 書いてくれて vs 書いてもらって?

Both くれる and もらう describe receiving something (a thing or an action), but from different viewpoints.

くれる: “someone gives me (something)” / “someone does (something) for me.”
- Focus is on the giver doing a favor toward the speaker.
- 先生は紹介状を書いてくれた
  → “The teacher wrote a letter of recommendation for me.”
もらう: “I receive (something)” / “I get (someone) to do (something).”
- Focus is on me receiving a benefit.
- 私は先生に紹介状を書いてもらった
  → “I got the teacher to write a letter of recommendation for me.” / “I received a recommendation letter from the teacher.”

In your sentence, 先生は…書いてくれて keeps the teacher as the topic and highlights the teacher kindly doing something for you.
If you used もらって, the sentence would naturally shift to something like:

私は先生に紹介状を書いてもらって、面接でそれを見せました。

which emphasizes me as the one who received the favor.

Why is 面接で used instead of 面接に?

The particle で marks the place where an action occurs.

面接でそれを見せました = “I showed it at the interview.”

で = “at / in / on (the place where something happens).”

If you used 面接に, it would sound more like “to the interview” (direction / destination), which is not the meaning here.
You’re not going to the interview in this clause; you are performing the action of showing in the context of the interview. So 面接で is correct.

Who is the subject of 見せました? Is it the teacher or “I”?

In Japanese, the subject is often omitted when it’s clear from context. Here, it’s most naturally understood to be “I”:

先生は私に紹介状を書いてくれて、
→ The teacher wrote me a recommendation letter (for my benefit), and then…
面接でそれを見せました。
→ (I) showed it at the interview.

Why not the teacher? Because:

The phrase 私に紹介状を書いてくれて strongly suggests I am the beneficiary and the one who will use the letter.
In real life, it’s normally the applicant (you), not the teacher, who takes the letter to the interview and shows it.

So even though 私 isn’t repeated, Japanese listeners will naturally interpret 見せました as “I showed (it).”

Why is それ used instead of repeating 紹介状?

それ is a pronoun meaning “that / it” and it refers back to something already known from context. Repeating 紹介状 would be grammatically fine but less natural because Japanese tends to avoid unnecessary repetition.

紹介状を見せました
is possible, but
それを見せました
sounds more natural once 紹介状 has already been mentioned.

So the flow is:

先生は私に紹介状を書いてくれて、
面接でそれを見せました。
→ “The teacher wrote me a recommendation letter, and I showed it at the interview.”

Why is the tense expressed only in 見せました and not in 書いてくれて?

This is a typical Japanese pattern for linking actions:

Non-final verbs: te-form (here, 書いてくれて)
Final verb: carries the tense and politeness (here, 見せました)

So:

書いてくれて (te-form)
見せました (past polite)

English would put both verbs in the past (“wrote” and “showed”), but Japanese marks the past tense just once at the end of the chain. The te-form implies the action happened as part of the same time frame, so 書いてくれて is also understood as past here.

Why is there no 私 in the second part, like 私は面接でそれを見せました?

Japanese often omits pronouns (私, あなた, etc.) when the subject is clear from context. The topic 先生は was mentioned, but then we have 私に…書いてくれて, which puts you in focus as the receiver and later user of the recommendation letter.

By the time we get to:

面接でそれを見せました。

it is already clear that you are the person who took the recommendation to the interview. Repeating 私は would be grammatically fine:

先生は私に紹介状を書いてくれて、私は面接でそれを見せました。

but it’s a bit more explicit than necessary and sounds slightly heavier. Omitting 私は sounds more natural and fluent.

How does 見せる work grammatically? Is it transitive or intransitive here?

見せる is a transitive verb; it takes a direct object marked with を.

それを見せました
→ “(I) showed it.”

Structure:

[object] を [verb]
- それ = object (“it,” the recommendation letter)
- を = object marker
- 見せました = “showed (politely, past).”

If you wanted to specify who you showed it to, you would add に:

面接官にそれを見せました。
→ “I showed it to the interviewer.”

What level of politeness is 書いてくれて、…見せました and in what situations is it appropriate?

見せました is the polite -ます form in the past tense.
書いてくれて is the te-form connecting to that polite verb.

Overall, the sentence is at the polite level (丁寧語), appropriate for:

talking to a teacher, interviewer, senior, or anyone you don’t know well
speaking in formal situations, such as interviews or polite conversations
writing in many formal contexts (though very formal writing might use slightly different phrasing)

A casual version between close friends might be:

先生が私に紹介状を書いてくれて、面接でそれを見せた。

dropping the ます and using た instead of ました.

Could I replace 先生は私に紹介状を書いてくれて with 私は先生に紹介状を書いてもらって? Does the meaning change?

Yes, you can, and the basic situation is the same, but the perspective shifts.

Original:

先生は私に紹介状を書いてくれて、面接でそれを見せました。
→ Topic = teacher; “The teacher wrote me a recommendation (for my benefit), and I showed it…”

Alternative:

私は先生に紹介状を書いてもらって、面接でそれを見せました。
→ Topic = me; “I got the teacher to write a recommendation letter for me, and I showed it…”

Both describe the same real‑world events, but:

The original focuses more on the teacher’s kind action.
The alternative focuses more on you receiving that favor.

Both are natural; which is better depends on what you want to highlight.