If, and when

English does the whole of conditional grammar with one word, and Japanese splits the job four ways, which is why "if" is the point where a competent beginner starts sounding wrong rather than merely simple. The good news is that between them the four cost exactly one new conjugation.

Four words, one new form

Take stock of what each of these actually demands of you.

と attaches to the plain form as it stands. 行いく gives 行いくと, 行いかない gives 行いかないと. Nouns and な adjectives insert だ: 学生がくせいだと. Nothing to learn.

たら is the た form plus ら, and you built the た form for free in the first lesson. 行いった gives 行いったら, 行いかなかった gives 行いかなかったら, 高たかかった gives 高たかかったら, 学生がくせいだった gives 学生がくせいだったら. Nothing to learn.

なら attaches to a plain verb or straight onto a noun with nothing between: 行いくなら, 学生がくせいなら, 安やすいなら. Nothing to learn.

ば is the only new conjugation in the lesson, and it is one vowel shift. For a group 1 verb, take the same final kana of the dictionary form that has now decided three other things for you, and move it to the え row, then add ば.

All nine endings, no exceptions, which is unusual enough in Japanese to be worth noticing. Group 2 replaces る with れば: 食たべれば, 見みれば. The two irregulars give すれば and 来くれば. Negatives use なければ, since every ない form is an い adjective: 行いかなければ. And い adjectives themselves swap い for ければ: 高たかければ, よければ. Nouns and な adjectives use なら, or the stiffer であれば, which is why なら and ば are historically the same word.

Example. Give all four conditional forms of 行いく and of 高たかい.

For 行いく: 行いくと, 行いけば, 行いったら, 行いくなら. The と form is the dictionary form untouched; the ば form moves く to け; the たら form comes off 行いった, which is the irregular て form 行いって with た; and なら sits on the dictionary form.

For 高たかい: 高たかいと, 高たかければ, 高たかかったら, 高たかいなら. Adjectives do all four as readily as verbs, which is easy to forget, and 高たかければ and 高たかかったら in particular are both very common.

Now you. Give all four conditional forms of 飲のむ, and the ば and たら forms of する and 来くる.

と: the automatic consequence

と says that whenever the first thing holds, the second follows. It is the conditional of natural law, machinery, arithmetic, road directions and habits, and it carries no sense of choice at all.

春はるになると、桜さくらが咲さく haru ni naru to, sakura ga saku when spring comes, the cherry trees blossom

このボタンを押おすと、切符きっぷが出でます kono botan o osu to, kippu ga demasu if you press this button, a ticket comes out

右みぎに曲まがると、駅えきがあります migi ni magaru to, eki ga arimasu if you turn right, there's the station

From that meaning follows the hardest restriction in the lesson, and it is genuinely absolute: the main clause of a と sentence cannot be a request, a command, an invitation or a statement of intention. An automatic consequence is not something anybody decides to do. このボタンを押おすと、切符きっぷを買かってください is not Japanese, and the fix is たら.

There is one further use of と that surprises learners. With a past main clause, と reports a discovery: 部屋へやに入はいると、電話でんわが鳴なっていた, "when I went into the room, the phone was ringing". Nothing is conditional about this. The と marks that the second thing was found to be the case on doing the first, which is the same automatic feel viewed from afterwards.

ば: the sufficient condition

ば puts the weight on the condition itself: provided this, that follows. It is the conditional of generalisations, proverbs and advice, and it is much commoner in writing than in casual speech.

安やすければ買かいます yasukereba kaimasu I'll buy it if it's cheap

練習れんしゅうすればするほど上手じょうずになる renshuu sureba suru hodo jouzu ni naru the more you practise, the better you get

That second pattern, ば plus the dictionary form plus ほど, is the standard way of saying "the more X, the more Y", and it is worth learning as a unit.

ば has a restriction of its own, weaker than と's and easy to state. If the ば clause is an action by the same person as the main clause, the main clause cannot be a request or command: 東京とうきょうに行いけば、電話でんわしてください is odd. If the ば clause is a state, an adjective, ある, a negative or a potential, the restriction lifts entirely: 時間じかんがあれば、手伝てつだってください is perfectly natural. Unlike the と rule, this one has exceptions that native speakers produce without hesitation, so treat it as a strong tendency rather than a law.

たら: the safe default

たら is the general-purpose conditional, and if you are ever unsure, it is the one to use. It covers "if" and "when" alike, it takes any kind of main clause including commands and requests, and it deals in particular occasions rather than general truths.

東京とうきょうに着ついたら、電話でんわしてください Tokyo ni tsuitara, denwa shite kudasai ring me when you get to Tokyo

雨あめが降ふったら、試合しあいは中止ちゅうしです ame ga futtara, shiai wa chuushi desu if it rains, the match is off

The one thing たら insists on is order: the main clause happens after the condition is satisfied. You arrive, then you ring. It rains, then the match is cancelled.

Like と, たら with a past main clause reports something found out, usually with a flavour of surprise: 家いえに帰かえったら、誰だれもいなかった, "when I got home there was nobody there". This is one of the highest-frequency uses of たら in ordinary conversation and it has nothing conditional about it at all.

Example. Correct this: 駅えきに着つくと、電話でんわしてください.

The main clause is a request, which と absolutely forbids, so replace it with たら: 駅えきに着ついたら、電話でんわしてください. Note that the fix is not a matter of politeness or nuance. と would make the sentence claim that arriving at the station automatically causes a phone call, and then attach an instruction to that claim, which is incoherent rather than merely unidiomatic. ば is also poor here, since arriving is an action by the same person, leaving たら as the only comfortable choice.

Now you. Which conditionals are possible in each, and which is best? (1) このボタンを押おす＿、音おとが出でます. (2) 時間じかんが＿、手伝てつだってください.

なら: somebody else's premise

なら is the odd one out and the one that most repays learning properly. It does not describe a condition being fulfilled. It picks up something already on the table, usually something the other person has just said, and builds on it: "if that's the case, then...".

日本にほんに行いくなら、京都きょうとがいいですよ nihon ni iku nara, Kyoto ga ii desu yo if you're going to Japan, Kyoto is worth it

The speaker is not wondering whether you will go. You said you were going, and なら takes that as given. This is why なら so often begins a reply, and why それなら, "in that case", is a complete conversational move.

Then comes the property that separates なら from the other three cleanly, and it is the single most useful test in this lesson. With なら, the main clause may happen before the condition. The other three all require the condition first.

日本にほんに行いくなら、ガイドブックを買かった方ほうがいい nihon ni iku nara, gaidobukku o katta hou ga ii if you're going to Japan, you should buy a guidebook (before you go)

日本にほんに行いったら、お土産みやげを買かってきてください nihon ni ittara, omiyage o katte kite kudasai when you go to Japan, buy me a souvenir (once you're there)

Same verb, same main-clause type, and the guidebook is bought in London while the souvenir is bought in Kyoto. Nothing but the choice of conditional carries that. If you can produce this pair reliably you have the distinction, and 方ほうがいい, "it would be better to", is a useful piece of advice-giving to pick up alongside it.

Example. Your friend says 来週らいしゅう大阪おおさかに行いきます. Tell them they should buy the ticket early.

大阪おおさかに行いくなら、早はやく切符きっぷを買かった方ほうがいいですよ. Two things force なら here and neither is optional. The trip is not in doubt: your friend has just announced it, so なら picks it up as given rather than treating it as a condition to be met. And the buying happens before the going, which rules out the other three outright. 行いったら切符きっぷを買かった方ほうがいい would advise buying a ticket after arriving in Osaka, which helps nobody.

Now you. Same friend, same trip. Ask them to try takoyaki, and say why the conditional has to change.

Choosing, and where the choice runs out

Four questions, asked in order, will get you the right conditional almost every time.

Is the condition something the other person just put on the table, or a premise you are adopting for the sake of argument? Use なら, and remember that it alone allows the main clause to come first in time.

Is the consequence automatic, invariable, mechanical or a matter of natural law? Use と, and check that the main clause is not a request, command, invitation or statement of will, because と refuses all four.

Are you making a generalisation, or stressing that the condition is enough on its own? Use ば, and if the condition is an action rather than a state, keep commands out of the main clause.

Otherwise use たら. It is the default for particular occasions, it takes any main clause, and it is what you will hear most in speech.

Now the honest part. These four overlap heavily, and in a great many sentences two or three are acceptable with only a shading of difference. Native speakers do not run a decision procedure; they have absorbed the tendencies. Regional variation is real, with Kansai speech leaning on たら noticeably more than Tokyo speech does. And the と restriction is the only rule here that is genuinely inviolable. Learn that one as a law, learn なら's independence from time order as the second most valuable fact, and treat the rest as preferences to be tuned by listening.

Vocabulary

Every sentence so far has been the speaker's own. The next lesson lets a clause belong to somebody else: what they said, what you think, and what you are not sure enough about to say outright.