watasi ha, nattou ni nori wo irete taberuno ga suki desu.

A useful breakdown is:

• 私は — as for me
• 納豆に海苔を入れて食べる — eat natto after adding nori to it / eat natto with nori added
• の — turns that whole action into a noun-like idea, eating natto with nori added
• が好きです — like / is liked

So the grammar is basically:

[action] の が 好きです
= I like [doing that action]

Very literally, it is something like:

As for me, eating natto with nori added is liked.

Natural English: I like eating natto with nori added.

は marks the topic.

So 私は means as for me or speaking of me. It sets up the sentence from your point of view.

In Japanese, this is very common when saying preferences:

• 私はコーヒーが好きです。
• 私は納豆が苦手です。

Also, in real conversation, 私 is often omitted if it is already obvious who is talking about themselves. So this sentence could naturally become:

納豆に海苔を入れて食べるのが好きです。

This follows the pattern:

A に B を 入れる
= put B into A

So here:

• 納豆に — into natto
• 海苔を — nori
• 入れて — put in / add, and then...

So 納豆に海苔を入れて means:

add nori to natto
or more literally
put nori into natto

A similar example:

• スープに塩を入れる — add salt to soup
• コーヒーに砂糖を入れる — add sugar to coffee

The て-form connects actions.

Here, 入れて食べる means:

• add it, then eat it
• or more naturally, eat it with that added

So the sequence is:

1. put nori into natto
2. eat it

In Japanese, this often sounds like a single combined action rather than two fully separate events. So 入れて食べる feels like eat it that way.

The の turns the verb phrase into a noun-like expression.

• 食べる — to eat
• 食べるの — eating / the act of eating

In this sentence, the whole phrase before の gets nominalized:

納豆に海苔を入れて食べるの
= eating natto with nori added

This is necessary because 好き takes a thing/idea that is liked. So Japanese turns the action into a noun-like unit first.

Yes. You could say:

納豆に海苔を入れて食べることが好きです。

This is grammatically correct.

But の often sounds more natural and conversational for likes, dislikes, and familiar actions. So:

• 食べるのが好きです — very common and natural
• 食べることが好きです — also correct, slightly more formal or abstract

Both work, but のが好きです is especially common in everyday Japanese.

In standard Japanese, 好き works differently from the English verb like.

It is more like an adjective or descriptive word meaning liked / pleasing, so the thing that is liked is commonly marked with が.

So:

• 納豆が好きです。 — I like natto.
• 映画を見るのが好きです。 — I like watching movies.
• 納豆に海苔を入れて食べるのが好きです。 — I like eating natto with nori added.

For learners, it is best to remember:

X が 好きです
= I like X

This is a very common Japanese feature: the object is often omitted when it is obvious.

Here, 納豆に海苔を入れて already tells you what food is being talked about. So when the sentence continues with 食べる, Japanese does not need to repeat the object.

It is understood as:

• add nori to natto
• eat that

If Japanese spelled everything out, it might become something like:

納豆に海苔を入れたものを食べるのが好きです。

But that is heavier and less natural in everyday speech. The original sentence is more natural.

Yes, the nuance is different.

• 納豆と海苔を食べるのが好きです
  = I like eating natto and nori.
  This could just mean you eat both together, without saying that the nori is added into the natto.

• 納豆に海苔を入れて食べるのが好きです
  = I like eating natto with nori added to it.
  This specifically describes the preparation/style of eating it.

So the original sentence is more specific.

Not usually.

Japanese often drops subjects when they are obvious from context. So in a normal conversation, many speakers would simply say:

納豆に海苔を入れて食べるのが好きです。

Adding 私は is still correct. It can help when:

• starting a new topic
• contrasting yourself with someone else
• making the sentence clearer for learners

No. It is not grammatically required.

The comma just adds a small pause or emphasis in writing:

私は、納豆に海苔を入れて食べるのが好きです。

You could also write:

私は納豆に海苔を入れて食べるのが好きです。

Both are fine. The meaning does not change.