kono uta ha watasi no ohako da.

Here, 十八番 is read おはこ, not じゅうはちばん.

This is one of those Japanese words whose reading is not obvious from the kanji. In everyday use:

• 十八番 = おはこ means one’s specialty, best thing, or signature piece
• 十八番 = じゅうはちばん would mean number 18

So in this sentence, you must read it as おはこ because that matches the meaning.

This is a cultural/historical expression.

十八番 / おはこ originally comes from 歌舞伎十八番, a famous set of eighteen kabuki plays associated with the Ichikawa acting line. Over time, the word came to mean a performer’s best-known piece or specialty.

So nowadays, 十八番 often means:

• the thing you do best
• your go-to song
• your signature move
• your forte

That is why it works naturally with 歌 in this sentence.

この means this and directly modifies the noun that follows it.

So:

• この歌 = this song

A good thing to remember is:

• この must be followed by a noun
• you cannot use この by itself

Compare:

• この歌 = this song
• これ = this one / this

So here, この is simply pointing out a specific song.

は marks the topic of the sentence.

So この歌は means something like:

• as for this song
• speaking of this song

The sentence is then making a comment about that topic: it is my specialty / signature song.

This is very common in Japanese. Instead of building the sentence around a strict English-style subject every time, Japanese often introduces a topic with は and then says something about it.

Here, の links 私 and 十八番.

• 私 = I / me
• 私の = my
• 私の十八番 = my specialty / my signature piece

So の is functioning like English my or of depending on context. In this sentence, it shows that the 十八番 belongs to me, in the sense of my best song.

Because 私 is not standing alone as a full sentence element here. It is part of the noun phrase 私の十八番.

In other words:

• 私 modifies 十八番 through の
• together they form one chunk: 私の十八番

So you do not need a separate particle after 私.

Compare:

• 私は学生だ。 = I am a student.
  Here 私 is the topic, so it takes は.

• 私の本だ。 = It is my book.
  Here 私 modifies 本, so it takes の.

Same pattern here.

だ is the plain form of the copula, roughly equivalent to is / am / are in this kind of sentence.

So:

• この歌は私の十八番だ。
• literally: As for this song, it is my specialty.

It gives the sentence a plain, direct tone.

A more polite version would be:

• この歌は私の十八番です。

So the difference is mainly level of politeness:

• だ = plain / casual
• です = polite

Yes. The sentence sounds plain and casual because it ends in だ.

That does not necessarily mean it is rude. It just means it is not in polite speech. You would commonly see this style in:

• casual conversation
• song lyrics
• novels
• dialogue
• informal writing

If you were speaking politely, you would usually say:

• この歌は私の十八番です。

Not exactly. 十八番 is closer to:

• specialty
• signature song
• the song I do best
• my go-to performance piece

It is not just something you like. It suggests ability, familiarity, and confidence.

So if someone says この歌は私の十八番だ, the nuance is more like:

• This is the song I’m best at singing
• This is my signature song

rather than simply I like this song.

Sometimes, yes, but the nuance changes.

• この歌は私の十八番だ。
  As for this song, it’s my signature song.

This version presents この歌 as the topic.

• この歌が私の十八番だ。
  This would put stronger focus on this song as the one that is your specialty, possibly in contrast to other songs.

So:

• は = topic, broader statement
• が = stronger focus or identification

In many everyday contexts, は sounds more natural here.

No. Normal Japanese writing does not usually put spaces between words like this.

The standard written form would be:

この歌は私の十八番だ。

Spaces are often added in textbooks or beginner materials to make the structure easier to see. They are a learning aid, not standard spelling.

Even if the meaning has already been shown, it helps to know the nuance. Natural English equivalents could be:

• This song is my signature song.
• This is the song I do best.
• This song is my specialty.
• This is my go-to song.

If the context is karaoke or singing, signature song is often the best match.