Learn Hardcore Japanese: nihon ni ikuno ha nikaime desu. - It is my second time going to Japan.

日本は makes Japan itself the topic.
Then 行くのが二回目です sounds like “As for Japan, the going (to it) is the second time,” which is not natural phrasing.

The natural thing to topicalize here is the whole activity 日本に行くの. So:

日本に行くのは二回目です。
As for going to Japan, this is the second time.

What is the basic structure of this sentence? How could I “word-for-word” break it down?

Breakdown:

日本 – Japan
に – to (direction/goal particle)
行く – go
の – nominalizer (turns “go to Japan” into a noun phrase)
は – topic marker (“as for …”)
二回目 – the second time
です – polite copula (“is”)

A very literal structure:

日本に　行くの　は　　二回目　です。
To-Japan going (as a thing) TOP second-time is.

Natural English:

“It’s my second time going to Japan.”

Where is “I” in this sentence? Why is there no word for I like 私?

Japanese often omits the subject when it is obvious from context.

In everyday conversation, if you say:

日本に行くのは二回目です。

the listener will assume:

(私は) 日本に行くのは二回目です。
(As for me,) going to Japan is the second time.

You could explicitly say 私は日本に行くのは二回目です, but it usually sounds unnecessary unless you need to contrast with someone else (e.g., “As for me, it’s my second time, but for him it’s his first.”).

Could I use が instead of は and say 日本に行くのが二回目です? What would be the difference?

日本に行くのが二回目です is grammatical, but the nuance is different.

日本に行くのは二回目です。
– Neutral statement; 日本に行くの is the topic you’re talking about.
– “As for going to Japan, it’s the second time.”
日本に行くのが二回目です。
– が marks “going to Japan” as the subject of “is the second time.”
– Often sounds like you’re answering an implied question such as
“Which thing is your second time?” → “Going to Japan is (my) second time.”

In most normal contexts where you’re just volunteering the information, は is more natural here.

What does 二回目 mean exactly? How is it formed?

二回目 means “the second time.”

It is built from:

二 – two
回 – counter for occurrences / times (as in “once, twice, three times”)
目 – suffix that makes it “-th in order” (ordinal marker)

So:

一回目 – the first time
二回目 – the second time
三回目 – the third time

In this sentence, 二回目 is functioning like an adjective/noun phrase describing the event: “(it) is the second time.”

What is the difference between 二回目 and 二度目? Could I say 日本に行くのは二度目です?

Yes, you can say:

日本に行くのは二度目です。

Both 二回目 and 二度目 mean “the second time.”

Nuances (very small and context-dependent):

回 is a very common, neutral counter for times/occurrences.
度 also means “time/occurrence,” and can sometimes feel a bit more written or slightly more formal/literary in some uses, but in this particular pattern (二度目です), it’s very common and natural in speech too.

For a learner, you can treat:

二回目です and 二度目です as practically interchangeable here.

What does the 目 part in 二回目 mean? Can I use it with other numbers/counters?

目 is an ordinal suffix: it turns “number + counter” into “Nth.”

Examples:

一回 → once
一回目 → the first time
三番 → number three (e.g., in a queue)
三番目 → the third one
二階 → second floor (as a level)
二階目 – the second floor up you count / second time you go up a floor (used in some specific contexts)

Common patterns:

一回目 / 二回目 / 三回目 – first/second/third time
一人目 / 二人目 / 三人目 – first/second/third person
一つ目 / 二つ目 – the first/second (thing)
一番目 / 二番目 – first/second (in order/ranking)

So 二回目 is literally “the second-in-order occurrence.”

Why is the verb 行く in dictionary form, not 行きます or some past/future form?

In 日本に行くの, 行く is inside a noun phrase, not acting as the main verb of the sentence.

The main verb (copula) of the sentence is です.
日本に行く is modifying の, creating the noun phrase “the act of going to Japan.”

For this kind of nominalization / relative clause, you generally use the dictionary form:

日本に行くの – going to Japan (as a thing)
日本に住むの – living in Japan (as a thing)

About tense:
The sentence is not directly saying “I will go” or “I went.” It is classifying the upcoming (or current) act of going as “the second time.” That’s why it doesn’t need a tense marker on 行く; the timing is understood from context (usually you are talking about the trip you’re about to take or are on now).

Why is the particle に used after 日本? Could I use へ instead?

に here marks the destination/goal: “to Japan.”

日本に行く – go to Japan

You can say:

日本へ行くのは二回目です。

へ also marks direction, especially “towards” somewhere. Differences:

に – more general and common for destinations; also used with many other functions (time, indirect object, etc.).
へ – focuses a bit more on direction/toward-ness; slightly more formal or written in some contexts.

In everyday speech, 日本に行く is more common and perfectly natural.

Could I say 日本に行くことは二回目です instead of 行くのは? What’s the difference between の and こと here?

You could say:

日本に行くことは二回目です。

It is grammatically correct and understandable. However, in casual natural speech, 日本に行くのは二回目です is more common.

Very roughly:

の as a nominalizer often feels a bit more conversational and concrete.
こと can feel slightly more abstract or formal, and appears a lot in set phrases (e.g., 〜することが大切です).

In many cases like this, の and こと are interchangeable, but native speakers often prefer の in everyday spoken Japanese for actions like this.

How can I say “It’s my first/third time going to Japan” using the same pattern?

You simply change the number part:

日本に行くのは一回目です。
It’s my first time going to Japan.
日本に行くのは二回目です。
It’s my second time going to Japan.
日本に行くのは三回目です。
It’s my third time going to Japan.

You can continue with 四回目, 五回目, etc.

You can also swap 回目 with 度目 similarly:

一度目, 二度目, 三度目 …

Can I make the sentence more explicit, like “This is my second time going to Japan,” by adding これ or something similar?

Yes, common natural variations include:

日本に行くのはこれで二回目です。
With this, it’s my second time going to Japan.
日本に行くのは今回で二回目です。
With this trip, it’s my second time going to Japan.

Here:

これで / 今回で explicitly anchor it to “this (occasion/trip).”
The core grammar 日本に行くのは〜回目です stays the same.