Learn Hardcore Japanese: watasi ha zyuusyohenkou no tetuduki ni hituyouna syorui wo kousikisaito de kakuninsimasita. - I checked the documents necessary for the address change procedure on the official site.

私は – topic: as for me / I
住所変更の手続きに必要な書類を – long noun phrase: the documents necessary for the procedure of changing my address (direct object)
公式サイトで – location of the action: on the official website
確認しました – verb: checked / confirmed

So the core pattern is:

私は　[長い名詞フレーズ] を　公式サイトで　確認しました。
I checked [long noun phrase] on the official website.

Inside the long noun phrase:

住所変更の手続き – procedure for address change
手続きに必要な書類 – documents necessary for the procedure

So 手続きに必要な is a relative clause that modifies 書類.

What does は after 私 do here? Could I omit 私?

は is the topic marker. 私は means as for me / I. It tells you that I am the one doing the action.

In natural conversation or writing, Japanese speakers would usually omit 私 here if it’s already clear who the subject is:

（私は）住所変更の手続きに必要な書類を公式サイトで確認しました。

Omitting 私 makes the sentence more natural and less stiff, unless:

You need to contrast I with someone else (e.g., 私は checked, but he didn’t), or
It’s the first time you’re clearly stating who did the action.

So yes, it’s grammatically fine and often more natural to drop 私.

What does 住所変更の手続き literally mean, and what is の doing there?

住所変更の手続き is:

住所変更 – address change (a noun made from 住所 “address” + 変更 “change”)
の – links two nouns (like of or for)
手続き – procedure / paperwork / formalities

Literally: “the procedure of address change” or more naturally, “the procedure for changing (one’s) address.”

Here の is the standard noun-connecting particle, used very broadly:

入学の手続き – procedure for enrollment
契約の手続き – procedure for a contract

Why is に used after 手続き in 手続きに必要な?

In 手続きに必要な, the に marks the target / context for which something is necessary.

X に必要な Y = Y that is necessary for X

So:

手続きに必要な書類
→ documents that are necessary for the procedure

Other similar patterns:

申し込みに必要な情報 – information necessary for the application
旅行に必要なお金 – money necessary for the trip

So に here is not marking location, but “for / required for.”

What kind of word is 必要, and why is it 必要な書類 instead of something like 必要書類?

必要（ひつよう） is a na-adjective (形容動詞). Na-adjectives modify nouns with な:

必要な書類 – necessary documents
便利な道具 – convenient tool

That’s why you see 必要な書類.

You can sometimes see 必要書類 as a compound noun (especially in headings, forms, or labels), but that’s more like a fixed term or title:

On a website section title: 必要書類 = “Required documents”

In a normal sentence, 必要な書類 is more grammatical and natural:

✅ 住所変更の手続きに必要な書類を確認しました。
Title/heading: 住所変更手続きに必要書類

So: 必要 is a na-adjective, and it needs な to modify 書類 in ordinary sentences.

How can I understand 手続きに必要な書類 more literally?

Break it down:

手続き – the procedure
に – for (in the sense of “required for”)
必要な – necessary
書類 – documents

So literally:
“documents that are necessary for the procedure.”

Putting it all together:

住所変更の手続きに必要な書類
= the documents that are necessary for the procedure of changing (my) address

Why is を used after 書類, and not は or が?

を marks the direct object of the verb:

書類を確認しました。
→ I checked the documents.

If you used は or が instead, you would change the focus:

書類は公式サイトで確認しました。
→ As for the documents, I checked them on the official site (maybe contrasting with something else you didn’t check there).
書類が公式サイトで確認しました。
→ This is wrong, because 書類 is not the subject doing the checking.

So here, 書類 is clearly the thing being checked, so it takes を.

Why do we use で after 公式サイト? Does it mean “at”, “in”, or “on” the website?

で marks the place where an action happens:

学校で勉強します。 – I study at school.
家でテレビを見ます。 – I watch TV at home.

For websites, Japanese usually uses で as well, even though English says “on a website”:

公式サイトで確認しました。
→ Literally: I checked (it) at the official site.
→ Natural English: I checked it on the official website.

So here で is “at/in/on (as the location of the action).”

What nuance does 確認しました have compared to 見ました or 調べました?

確認しました
- From 確認する – to confirm, to check (to make sure)
- Implies you checked to make sure something was correct, complete, or as expected.
- More formal / businesslike.
見ました
- From 見る – to see / to look
- Just means you looked at something; no strong nuance of verifying.
調べました
- From 調べる – to investigate / to look up
- Emphasizes searching for / investigating information.

In this context:

公式サイトで必要な書類を確認しました。
= I checked (to make sure of) which documents are required on the official website.

This sounds appropriate for procedures and official information.

Can I say 確認した instead of 確認しました? What changes?

Yes, grammatically:

確認した – plain past
確認しました – polite past (ます form)

Meaning is the same (“checked / confirmed”), but:

確認しました is polite and suitable for talking to strangers, colleagues, in emails, etc.
確認した is casual/plain, used with friends, in diaries, in novels’ narrative, etc.

So you choose based on politeness level, not meaning:

To your friend: 住所変更の手続きに必要な書類を公式サイトで確認したよ。
In a polite email: 住所変更の手続きに必要な書類を公式サイトで確認しました。

Can I move 公式サイトで to another position in the sentence?

Yes. Japanese word order is fairly flexible as long as particles stay attached to the right words. All of these are natural:

私は住所変更の手続きに必要な書類を公式サイトで確認しました。
私は公式サイトで住所変更の手続きに必要な書類を確認しました。
住所変更の手続きに必要な書類を公式サイトで確認しました。 (dropping 私は)

Often, something like (2) or (3) with a comma is used:

公式サイトで、住所変更の手続きに必要な書類を確認しました。

The most fixed part is that the verb usually comes at the end. Other phrases can move around as long as the particles (は, を, で, に, etc.) stay attached correctly.

How do you read each part of this sentence?

Readings (in romaji):

私 – わたし – watashi
は – particle – wa
住所 – じゅうしょ – jūsho
変更 – へんこう – henkō
住所変更 – じゅうしょへんこう – jūsho henkō
の – particle – no
手続き – てつづき – tetsuzuki
に – particle – ni
必要な – ひつような – hitsuyō na
書類 – しょるい – shorui
を – particle – o (often written wo in romaji)
公式サイト – こうしきサイト – kōshiki saito
で – particle – de
確認しました – かくにんしました – kakunin shimashita

Full reading:

わたしはじゅうしょへんこうのてつづきにひつようなしょるいをこうしきサイトでかくにんしました。

Could I say something like 住所変更に必要な手続きの書類 instead? Does it change the meaning?

You can rearrange it, but the nuance changes.

Original:

住所変更の手続きに必要な書類
→ Focus: 書類 that are necessary for the procedure.
→ documents necessary for the address-change procedure

Alternative:

住所変更に必要な手続きの書類
- 住所変更に必要な手続き – the procedures necessary for changing the address
- の書類 – the documents of/about those procedures

This sounds more like:

the documents of the procedures that are necessary for changing your address

It shifts the main relationship:

Original: 書類 ←necessary for→ 手続き
Alternative: 手続き ←necessary for→ 住所変更

The original sentence is the most natural and standard way to say “the documents required for the address change procedure.” The alternative is grammatical but more convoluted and less natural here.