Title

The simplest sentence structure in most languages is "X is Y." This sentence structure in grammar is called either "copular sentence" or "noun-predicate" sentence (meishi-jutsugo-bun 名詞述語文). The Japanese copula verb is used a lot, so just learning about this single grammar item will get you making sentences in no time. First, let's cover some basic grammar terminology to make reading this chapter a lot easier.

• Subject: The person/thing that performs the action or exhibits the description found in the predicate.
• Predicate: The part of a sentence that makes a statement about the subject. 
• Copula: A word used to link the subject and predicate of a sentence.
  
• Noun: In its most basic definition, a word that refers to a person, place, thing, event, substance, or quality. 
  
• Auxiliary: An ending that helps construct verbal conjugations.
• Plain Speech: Jōtaigo 常体語 refers to the entire plain speech register in Japanese grammar.
• Plain Style: Jōtai 常体 is the plain speech rendition of any given phrase.
• Plain Form: Kihonkei 基本形 is the basic form of any given phrase: a.k.a, its plain form.

In English, the copula verb is “to be,” and it manifests itself in various forms such as “is,” “are,” “was,” etc. 

i. The dog is a German shepherd.
ii. My husband is a banker.
iii. Apples are fruits.
iv. It was a fossil.
v. A bat is not a bird.

Similarly, Japanese has its own copular verb, which in turn has its own various forms, but Japanese doesn't conjugate for the same things as English. 

In English, there are three tenses: past, present, and future. As their names suggest, the past tense refers to an event/state which occurred in the past, the present tense refers to a current event/state, and the future tense refers to an event/state that hasn’t yet realized. Affirmation is positively stating that something is so. Negation is rejecting a premise.

Japanese only has two tenses: non-past and past tense. The non-past tense encompasses both the concepts of the English present tense and future tense. The past tense corresponds to the past tense, but the ending for it in Japanese doesn't always correspond to it. How tense is marked will occasionally differ from English, but we'll get to those situations later.

As was introduced in the preface of いまび , speech register affects every aspect of Japanese phrasing. The majority of form changes the copula may take are, in fact, based on speech register. To begin covering these various forms, we will start off by looking at the copula's most basic form: its plain form da だ. This word attaches directly after nouns and can be translated as "is," "are," "to be," "will be," etc. 

Because the verb of a Japanese sentence must always be at the end of a sentence, we can’t simply insert da だ between “X” and “Y.” “X” remains at the start of the sentence, and the sentence ends in “Yだ.”  To complete the sentence, we will insert the particle wa は in between X and Y. In Lessons 11-12, we’ll learn about the sort of nuances that are expressed with this particle as well as what else can be between X and Y. For now, though, our goal is just to master “XはYだ.”

The non-past form (hikakokei 非過去形・kihonkei 基本形) of verbs may appear difficult just from the name, but when the context has nothing to do with a future date, the non-past form is simply understood as the present tense (genzai jisei 現在時制). If, however, you have a sentence like "Tomorrow there will be rain," you still use the non-past form of だ . This is the future tense (mirai jisei 未来時制) interpretation. Either way, the non-past form of the copula is だ. In English, there is a distinction in grammar between the "to..." (base) form of a verb and those you use in other conjugations, but this is not the case in Japanese.   

Terminology Note: Jisei 時制 means "tense" whereas -kei 形 means "form," and since Japanese doesn't have separate forms for the present tense and future tense, the hikakokei 非過去形 is the only 'form' that actually exists. Nonetheless, this same form is often referred to as genzaikei 現在形 (present tense form) and miraikei 未来形 (future tense form) depending on its interpretation.  

Now, time for some example sentences (reibun 例文)!

Non-Past: Present

1. あれは寺だ。
Are wa tera da.
That over there is a Buddhist temple.

2. それは嘘だ。
Sore wa uso da.
That is a lie.

3. これは事実だ。
Kore wa jijitsu da.
This is the truth.

4. 僕は学生(だ)。
Boku wa gakusei (da).
I’m a student.

5. キムは韓国人(だ)。
Kimu wa kankokujin (da).
Kim is Korean.

6. 書道は芸術だ。
Shodō wa geijutsu da.
Calligraphy is art.

7. 彼はバカだ。
Kare wa baka da.
He’s an idiot.

One nuance that can be noted from these examples without being too specific is that だ is very direct. In general, Japanese speakers are more likely to be this direct when writing more so than when they're speaking. Especially when talking about interpersonal things, in casual conversation where one might utilize Ex. 4 and  Ex. 5, it would be more natural to drop だ altogether. When it is dropped from a sentence, though, it is believed to still be there structurally. As for Ex. 7, it sounds more like some quick remark or title on a webpage.  

Non-Past: Future

As can be deduced from the examples, English also shows fluctuation between "is" and "will be" in certain contexts, especially when referencing established dates.

8. 元日は月曜日だ。
Ganjitsu wa getsuyōbi da.
New Year’s Day [is/will be] on Monday.

9. 開始日は明日だ。
Kaishibi wa ashita da.
The start date [is/will] be tomorrow.

10. パーティーは夜だ。
Pātii wa yoru da.
The party [is/will be] at night.

Omitting “X”

In Japanese, the subject and even the topic of discussion are often dropped from the sentence. This is especially the case when the subject of the sentence is “it" in English. 

11. 明日だ。
Ashita da.
[It’s/it'll be] tomorrow.

12. あ、猫だ！
A, neko da.
Ah, (it’s) a cat!

13. 時間だ。
Jikan da.
(It’s) time.

As already seen above, the copula だ is usually dropped altogether. You may hear heightened intonation to match the various emotions someone might be trying to convey with a more direct speech style. Not dropping だ is more indicative of a firmer, and to some speakers, a more masculine means of talking. It's worth noting that there are actually contexts in which "to be" is dropped in English, and that can be carried over into Japanese.

14. あ、ピカチュウ（だ）！
A, Pikachū (da)!
Ah, (it’s) (a) Pikachu!

15. 開始（だ）！
Kaishi (da)!
Start!

16. 動物園（だ）！
Dōbutsuen (da)!
(It’s) [a/the] zoo!

17. 引き分け（だ）！
Hikiwake (da)!
(It’s a) draw!

18. あ、カワウソ（だ）！
A, kawauso (da)!
Ah, (it’s) an otter!

To express past tense (kakokei 過去形) with the copula da だ, you conjugate it to datta だった. As you learn more, you will see that -TA stands for -ed in anything that conjugates. The reason why there is a second "t" is because だ itself is a contraction, which also means that だった is a conjugation. Their uncontracted forms don't pertain to us just yet, so for now know that だった equates to both "was" and "were." 

Conjugation Recap

Are wa Furansugo datta.
That was French.

20. ここは学校だった。
Koko wa gakkō datta.
This here was a school.

21. 正解はAだった。
Seikai wa ei datta.
The correct answer was A.

22. 昨日は水曜日だった。
Kinō wa suiyōbi datta.
Yesterday was Wednesday.

23. 彼は子供だった。
Kare wa kodomo datta.
He was a child.

Grammar Note: The past tense form need not always be interpreted literally. Ex. 23 implies that a male individual happened to be a child and is said as a remember to oneself and/or to others. 

"Negative-past" (kako-no-hiteikei 過去の否定形) refers to "wasn't/weren't," and the way to form this is by changing the negative auxiliary -nai ない to nakatta なかった. The reason why ない becomes なかった lies in the fact that it conjugates as an adjective, which we'll learn more about in Lesson 12. The resulting de wa nakatta ではなかった can then be contracted to ja nakatta じゃなかった in casual speech.

Conjugation Recap

Although learning how to say "is/are," "was/were," and "wasn't/weren't" may not seem worth all the time and effort to read through the text and examples, we only covered copula expressions in one register, yet we still learned quite a bit how register helps shape the sort of nuances the speaker may make. In our next lesson, we will take things a step further by learning about copula expressions in polite speech. This is the typical starting point, but when considering complexity, one will find that using polite speech 'correctly' is actually trickier.