No matter how long or complex a Japanese sentence is, it always belongs to one of four sentence types. The classification method is surprisingly simple: look at the end.
What Counts as a "Sentence"?
A complete Japanese sentence must have two things:
- Subject: noun + は / が
- Predicate: placed at the end, determines the sentence type
A subject without a predicate, or a predicate without a subject — neither qualifies as a complete sentence.
What Can Be a Predicate?
| Word type | Can it be a predicate? | Condition |
|---|---|---|
| Verb | Yes | Placed directly at the end |
| I-adjective | Yes | Placed directly at the end |
| Na-adjective | Yes | Must add だ / です |
| Noun | Yes | Must add だ / です |
Key point: Na-adjectives and nouns cannot stand alone at the end of a sentence — they must be followed by だ (plain) or です (polite) to function as a predicate.
Quick Judgment Practice
Which of these are complete sentences?
| Content | Complete sentence? | Reason |
|---|---|---|
| 東京で食べる | No | Has a verb predicate but no subject (は/が) |
| 田中さんが行く | Yes | Subject (田中さんが) + verb predicate (行く) |
| リンゴがおいしい | Yes | Subject (リンゴが) + i-adjective predicate (おいしい) |
| あの人は有名 | No | 有名 is a na-adjective with no だ / です |
| あの人は有名だ | Yes | Subject (あの人は) + na-adjective predicate (有名だ) |
| 部屋はここにある | Yes | Subject (部屋は) + verb predicate (ある) |
The Four Sentence Types
Based on the predicate's word type, Japanese sentences fall into four categories:
1. Verb Sentence (動詞句)
The predicate is a verb.
田中さんは学校へ行く。 → Tanaka goes to school.
行く (verb) at the end → verb sentence.
2. I-Adjective Sentence (形容詞句)
The predicate is an i-adjective.
このラーメンはおいしい。 → This ramen is delicious.
おいしい (i-adjective) at the end → i-adjective sentence.
3. Na-Adjective Sentence (形容動詞句)
The predicate is a na-adjective + だ/です.
教室は静かだ。 → The classroom is quiet.
静かだ (na-adjective + だ) at the end → na-adjective sentence.
4. Noun Sentence (名詞句)
The predicate is a noun + だ/です.
今日はいい天気だ。 → Today is nice weather.
天気だ (noun + だ) at the end → noun sentence.
Summary Table
| Sentence type | Predicate | Example |
|---|---|---|
| Verb sentence | Verb | 明日は学校へ行く。 |
| I-adj sentence | I-adjective | このケーキはおいしい。 |
| Na-adj sentence | Na-adj + だ/です | 教室は静かだ。 |
| Noun sentence | Noun + だ/です | 今日はいい天気だ。 |
No matter how long the sentence is, look at the final word to classify it.
ので: Expressing Cause with Plain Form
Now that you know plain form and sentence classification, you can immediately use an important conjunctive particle: ので.
ので expresses cause and effect, equivalent to "because... so..." in English.
Basic Rule
ので must be preceded by plain form.
| Word type | How to connect with ので | Example |
|---|---|---|
| Verb | Plain form + ので | 風邪をひいたので、学校を休みます。 |
| I-adjective | Plain form + ので | 高いので、買えません。 |
| Na-adjective | Plain だ → な + ので | 静かなので、よく眠れます。 |
| Noun | Plain だ → な + ので | 祭りなので、人が多い。 |
Note: Na-adjectives and nouns are special — instead of だので, the だ changes to な before ので.
Why Does だ Become な?
When the plain affirmative form of a na-adjective or noun is followed by ので, だ transforms into な:
- 静かだ → 静かなので
- 祭りだ → 祭りなので
This な originates from だ. Just remember: na-adjectives and nouns use な before ので.
Example Breakdown
1. 風邪をひいたので、学校を休みます。
| Part | Analysis |
|---|---|
| 風邪をひいた | Caught a cold (verb, plain past) |
| ので | So / because |
| 学校を休みます | Take a day off from school (polite) |
→ Because I caught a cold, I'm taking a day off from school.
2. 祭りなので、人が多い。
| Part | Analysis |
|---|---|
| 祭りな | It's a festival (noun + な) |
| ので | So / because |
| 人が多い | There are many people (i-adj sentence) |
→ Because it's a festival, there are many people.
3. 図書館に人がたくさんいたので、入れませんでした。
→ Because there were many people in the library, I couldn't get in.
ので Allows Mixed Formality
Notice in the examples above: the part before ので is plain form (ひいた), while the part after is polite form (休みます). This is perfectly normal — ので only requires plain form before it; what comes after is up to you.
Summary
- Complete sentence = subject (noun + は/が) + predicate
- Four types based on predicate: verb, i-adjective, na-adjective, noun sentence
- Na-adjectives and nouns need だ (plain) or です (polite) to be predicates
- ので expresses cause-and-effect; preceded by plain form
- Na-adjectives / nouns use な (not だ) before ので
Practice
Q1. What sentence type is 「この本はおもしろい。」?
Show Answer
I-adjective sentence.
おもしろい is an i-adjective functioning as the predicate → i-adjective sentence.
Q2. Connect these two sentences using ので: 「明日は休みだ」+「どこかへ行きたい」
Show Answer
明日は休みなので、どこかへ行きたい。
休み is a noun; when connecting to ので, だ becomes な → 休みなので.
Q3. Is 「あの人は有名」a complete sentence? If not, how do you fix it?
Show Answer
It is not a complete sentence.
有名 is a na-adjective and needs だ (plain) or です (polite) after it to function as a predicate.
Fix: あの人は有名だ。 or あの人は有名です。