Pretending to be a witch, Chinatsu casts a spell to turn her older brother, Kei, into a doughnut.

圭(けい)：

「そうだ　私(わたし)がドーナツだ」

“That’s right, I am a doughnut!”

In this response, Kei couples the subject 私(わたし) (I) to the noun ドナッツ (doughnut), stating “I am a doughnut.” だ is used as the coupler, similar to the word “is” (or “am”) in English.

In formal speech, です is used as the coupler.

As instructor of the school diving club, one of Katori’s first lessons is about water pressure. Following a demonstration of equalizing pressure in one’s ears with the pressure of the water, she finishes the lesson by giving the name of this concept.

火鳥(かとり)：

「それが耳(みみ)抜(ぬ)きです」

“That is equalizing the pressure in your ears with external pressure.”

That concept is bundled into the pronoun それ (that), and is then coupled with the name (noun) 耳(みみ)抜(ぬ)き. Because she is speaking formally, rather than だ, the formal ending です acts as the coupler.

In more casual speech, the だ may be left off.

Kei visited his friend Nao’s family shop to pick up an order. After hearing from Kei about having a relative move in, Nao sums it up in her response.

なお：

「へー　親戚(しんせき)が居候(いそうろう)」

“I see. So your relative’s staying over.”

The subject, 親戚(しんせき) (relative), is coupled with the noun 居候(いそうろう) (free lodger). In this case, the coupling だ is unspoken.

When Kujou first sees Victorica, he first thinks he’s seeing a doll.

久城(くじょう)：

「人形(にんぎょう)だ」

“A doll.”

Since he’s thinking, there’s no need to state the subject who looks like a 人形(にんぎょう) (doll). It’s the girl he’s looking right at.

Sacchan receives a text message from her mother.

さっちゃん：

「大変(たいへん)だ！！」

“An emergency!”

In this situation, the subject is both unstated, and unknown. To find out what the subject is that is 大変(たいへん) (a disaster), Sacchan must visit her mother and ask about it.

While waiting for his girlfriend, Shingo is approached by an elderly couple. They’re looking for a nearby restaurant, and he offers to guide them there. When they arrive, he points to their destination.

しんご：

「あちらです」

“It’s over that way.”

There is no need to state the subject, to say what is あちら (over there). It’s clear from context that over there is the restaurant the elderly couple is looking for. Because he is speaking to his elders, Shingo uses the polite です rather than だ.

After getting mad at Asuka Jr., Meimi throws her book bag at him, and shouts:

芽美(めいみ)：

「バカアッ！！」

“Idiot!”

There’s no need to name the subject. Asuka Jr. very well knows who’s being called バカ. As does everyone else in class.