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Conditionals (条件文) - 完全ガイド

Zero Conditional (ゼロ条件文) — 事実・法則

Structure

If + present simple, present simple

Usage

Scientific facts, general truths
Things that are always true

Examples

If you heat water to 100°C, it boils.
If you mix red and blue, you get purple.
If I eat too much, I feel sick.
If you don't water plants, they die.
If it rains, the ground gets wet.
If you press this button, the machine starts.
If babies are hungry, they cry.
If you freeze water, it turns into ice.
If I drink coffee at night, I can't sleep.
If you drop something, it falls.

忘れがちなポイント

"when" と交換可能なことが多い: When you heat water to 100°C, it boils.
個人的な習慣にも使える: If I'm tired, I go to bed early.

First Conditional (第一条件文) — 現実的な未来の可能性

Structure

If + present simple, will + base verb

Usage

Real, possible future situations
Predictions, promises, warnings, threats

Examples

If it rains tomorrow, I will stay home.
If you study hard, you will pass the exam.
If she doesn't hurry, she will miss the train.
If I get the job, I will move to Osaka.
If you eat that, you will get sick.
If we leave now, we will arrive on time.
If he calls, tell him I'm busy. (命令文も可)
If you need help, I will be there.
If the weather is nice, we will go to the beach.
If I have time, I will cook dinner tonight.
If you don't stop that, I will call the police. (警告)
If you finish early, you can leave. (can も可)

Variations — will 以外も使える

If it rains, we might go to the museum. (かもしれない)
If you're good, you can have dessert. (許可)
If he's late, we should start without him. (提案)
If you see him, tell him to call me. (命令)

忘れがちなポイント

If節にwillは原則使わない!
✗ If it will rain, I will stay home.
✓ If it rains, I will stay home.
ただし「意志」を表すwillはOK:
If you will wait a moment, I'll check. (待っていただけるなら — 丁寧)
unless = if not:
Unless you study, you won't pass. = If you don't study, you won't pass.

Second Conditional (第二条件文) — 仮定・非現実

Structure

If + past simple, would + base verb

Usage

Imaginary/unlikely present or future situations
Dreams, hypothetical questions
Advice (If I were you...)

Examples

If I won the lottery, I would travel the world.
If I were you, I would accept the offer. (wereが正式 — wasも口語OK)
If I had more time, I would learn French.
If she spoke English, she would get a better job.
If I could fly, I would go everywhere.
If I lived in New York, I would eat pizza every day.
If he weren't so lazy, he would succeed.
If I didn't have to work, I would sleep all day.
What would you do if you found a million dollars?
If animals could talk, what would they say?
If I were taller, I would play basketball.
If I knew the answer, I would tell you.

I were vs I was

If I were you... (フォーマル、伝統的に正しい — 仮定法)
If I was you... (口語ではOK、でも試験では were が安全)
"If I were" は仮定法過去 (subjunctive) の名残

忘れがちなポイント

過去形を使うが、意味は「今」の仮定!
If I had a car... (今車を持っていたら — 実際は持っていない)
could = 「できたら」のバリエーション:
If I could speak Chinese, I would work in Shanghai.

Third Conditional (第三条件文) — 過去の非現実

Structure

If + past perfect, would have + past participle

Usage

Imaginary past situations (things that didn't happen)
Regrets, criticism, different outcomes

Examples

If I had studied harder, I would have passed the exam. (もっと勉強していたら受かっていた)
If she had left earlier, she wouldn't have missed the train.
If it hadn't rained, we would have gone to the beach.
If I had known you were coming, I would have baked a cake.
If he had listened to my advice, he wouldn't have failed.
If they had saved more money, they could have bought a house.
If I had been born in America, I would have spoken English natively.
If we hadn't gotten lost, we would have arrived on time.
If she had told me the truth, I would have forgiven her.
If I had taken that job, my life would have been completely different.
What would you have done if you had been in my situation?
If he hadn't called me, I would never have known.

忘れがちなポイント

口語での短縮:
If I'd known → "If I'd known..." (had の短縮)
I would have → "I would've" or "I woulda" (超口語)
could have / might have も使える:
If I'd known, I might have helped. (助けたかもしれない)
If she'd tried, she could have succeeded. (成功できたかもしれない)

Mixed Conditionals (混合条件文) — 上級

Type 1: 過去の条件 → 現在の結果

If + past perfect, would + base verb

過去にこうだったら、今こうだろう

If I had studied medicine, I would be a doctor now.
If she hadn't moved to Japan, she wouldn't speak Japanese.
If I had taken that job, I would be living in London now.
If he hadn't broken his leg, he would be playing in the game today.
If they had invested in Bitcoin early, they would be millionaires.

Type 2: 現在の条件 → 過去の結果

If + past simple, would have + past participle

今こういう人だったら、あの時こうしていただろう

If I were braver, I would have asked her out.
If he weren't so stubborn, he would have apologized.
If I spoke French, I would have understood the movie.
If she weren't so kind, she wouldn't have helped them.
If I liked spicy food, I would have tried that dish.

Other Conditional Structures

Suppose / Supposing / Imagine

Suppose you won the lottery, what would you do?
Imagine you could live anywhere, where would you choose?

As long as / Provided (that) / On condition that

You can go as long as you finish your homework.
I'll lend you the money provided that you pay me back.

Even if — たとえ〜しても

Even if it rains, we'll go. (雨でも行く)
Even if I had known, I wouldn't have helped. (知っていても助けなかった)

But for / If it weren't for / If it hadn't been for — 〜がなかったら

But for your help, I would have failed. (あなたの助けがなかったら)
If it weren't for music, life would be boring.
If it hadn't been for the doctor, he would have died.

I wish / If only — 第二・第三条件と同じ構造

I wish I were taller. (今の仮定 — Second Conditional の if 節と同じ)
I wish I had studied harder. (過去の後悔 — Third Conditional の if 節と同じ)
If only I could speak English fluently!

Common Mistakes by Japanese Speakers

✗ If I will have time, I will call you. → ✓ If I have time, I will call you.
✗ If I would be rich, I would buy a house. → ✓ If I were rich, I would buy a house.
✗ If I studied harder, I passed the exam. → ✓ If I had studied harder, I would have passed the exam.
✗ If I would have known, I would have told you. → ✓ If I had known, I would have told you.
日本語の「もし〜なら」は全条件文に対応するので、状況で判断する必要がある

Practice Exercises

If I ___ (be) you, I ___ (not/do) that. → were, wouldn't do
If she ___ (study) harder, she ___ (pass) the exam last week. → had studied, would have passed
If it ___ (rain) tomorrow, we ___ (cancel) the picnic. → rains, will cancel
If I ___ (know) her number, I ___ (call) her. → knew, would call
If he ___ (not/eat) so much, he ___ (not/feel) sick now. → hadn't eaten, wouldn't feel (mixed)
Water ___ (boil) if you ___ (heat) it to 100°C. → boils, heat
If I ___ (have) wings, I ___ (fly) to Hawaii. → had, would fly
If they ___ (arrive) earlier, they ___ (see) the opening ceremony. → had arrived, would have seen