sinsatuken wo wasureta node, namae to denwabangou wo kakukoto ni narimasita.

診察券 means a clinic card, patient card, or medical registration card.

It is the card you bring to a clinic or hospital in Japan so the staff can look up your record. It is made from:

• 診察 = medical examination / consultation
• 券 = ticket / card / voucher

So 診察券を忘れた means I forgot my clinic card.

を marks the direct object of the verb.

In 診察券を忘れた, the thing being forgotten is 診察券, so it takes を.

• 診察券を忘れた = forgot the clinic card

This is the same pattern as:

• 財布を忘れた = forgot my wallet
• 宿題を忘れた = forgot my homework

忘れた is the plain past form of 忘れる.

Here it means forgot. Since the forgetting happened before the next action, the past form is natural.

The sentence structure is:

• 診察券を忘れたので = because I forgot my clinic card...
• 名前と電話番号を書くことになりました = it ended up being decided that I would write my name and phone number

Also, in Japanese, a clause before ので is usually in plain form, even if the main sentence is polite. So this combination is normal:

• 忘れたので、... なりました

Both ので and から can mean because, but the nuance is a little different.

• ので sounds softer, more neutral, and a bit more explanatory
• から can sound more direct or more subjective

So:

• 診察券を忘れたので、... = Because I forgot my clinic card, ...
• 診察券を忘れたから、... = Because I forgot my clinic card, ...

In this kind of situation, especially when explaining circumstances politely, ので sounds very natural.

The comma helps separate the reason clause from the main result clause.

• 診察券を忘れたので、 = because I forgot my clinic card,
• 名前と電話番号を書くことになりました。 = I ended up having to write my name and phone number.

The comma is very common here, especially when the first part is a little long. It improves readability, but punctuation in Japanese can be somewhat flexible.

Because 名前と電話番号 is treated as one combined object: name and phone number.

Then を marks the whole thing as what is being written.

So:

• 名前と電話番号を = name and phone number + object marker

This is similar to:

• パンと牛乳を買いました = I bought bread and milk
• 住所と名前を書いてください = Please write your address and name

You do not normally repeat を after each noun in a simple list like this.

こと turns a verb into a noun-like expression.

• 書く = to write
• 書くこと = writing / the act of writing / to write as a thing

This is necessary because the pattern ことになる uses a verb phrase turned into a noun-like unit.

So:

• 名前と電話番号を書くこと = the act of writing my name and phone number

ことになる means it has been decided that..., it turns out that..., or I ended up having to..., depending on context.

Here, 名前と電話番号を書くことになりました means something like:

• It was decided that I would write my name and phone number
• I ended up having to write my name and phone number

This expression often suggests that the result comes from circumstances, rules, or someone else’s decision, rather than a personal voluntary choice.

In this sentence, the idea is not simply I decided to write it. It is more like:

• Because I forgot my clinic card, the situation became one where I had to write my name and phone number.

This is a very important difference.

• ことにしました = I decided to do it
• ことになりました = it was decided / it ended up that I would do it

So:

• 名前と電話番号を書くことにしました = I decided to write my name and phone number
• 名前と電話番号を書くことになりました = It was decided that I would write my name and phone number / I ended up having to write it

In your sentence, なりました is used because the action is caused by the situation at the clinic, not just by the speaker’s independent decision.

Because the sentence is describing what happened in that situation.

• ことになります would sound like a present/future statement: it will be decided / it will turn out that...
• ことになりました means it became the case that... or it ended up that...

Since the speaker is talking about what happened after forgetting the clinic card, the past tense is natural.

Japanese often leaves out the subject when it is clear from context.

In this sentence, English might say:

• Because I forgot my clinic card, I ended up having to write my name and phone number.

But in Japanese, the I does not need to be stated if the listener already understands who is involved.

This is extremely common in Japanese. The subject is often omitted unless it needs emphasis or clarification.

It is basically polite overall, and the mix is normal.

• 忘れた is plain past
• なりました is polite past

This is not strange. In Japanese, subordinate clauses often use plain forms, even when the main clause is polite.

So this pattern is very common:

• 理由の部分: plain form
• 文末: polite form

For example:

• 熱があったので、会社を休みました。
• 電車が遅れたので、遅刻しました。

So 忘れたので、... なりました sounds natural.

Yes, but it would mean something a little different.

• 名前と電話番号を書くことになりました = I ended up having to write my name and phone number
• 名前と電話番号を書いてくださいと言われました = I was told, Please write your name and phone number

The first one focuses on the resulting situation. The second one focuses on the staff’s explicit instruction.

Both could fit the same real-life situation, but they present it differently.

A fairly literal translation would be:

Because I forgot the clinic card, it became the case that I wrote my name and phone number.

But that sounds unnatural in English. A more natural translation would be:

• Because I forgot my clinic card, I had to write my name and phone number.
• Since I forgot my clinic card, I ended up writing my name and phone number.

This is a good example of how Japanese grammar and English grammar do not always match word for word.

It breaks down like this:

• 診察券を忘れた = forgot my clinic card
• ので = because / since
• 名前と電話番号を = name and phone number
• 書くことに = into the matter of writing / to the act of writing
• なりました = it became / it was decided / I ended up

So the full structure is:

[Reason] 診察券を忘れたので、
[Result] 名前と電話番号を書くことになりました。

In natural English:

Because I forgot my clinic card, I ended up having to write my name and phone number.