In short

Japanese has four conditionals; these are the two you need first. 〜たら is the all-purpose “if / when” - just add ら to the plain past form you already know (行った -> 行ったら). 〜と attaches to the dictionary form and means “whenever X, Y automatically follows” - natural law, machines, directions, habits. The one rule that decides everything: after と you may not ask, invite, order or announce a decision; after たら you may.

The rule

Step 1 - Build たら: plain past + ら. There is no new conjugation to learn. Take the た-form from lesson 17 and stick ら on the end. Verbs whose past ends in だ keep the voicing: 飲んだ -> 飲んだら nondara.

• 行く -> 行った -> 行ったら ittara
• 食べる -> 食べた -> 食べたら tabetara
• 高い -> 高かった -> 高かったら takakattara
• 静かだ -> 静かだった -> 静かだったら shizuka dattara
• 学生だ -> 学生だった -> 学生だったら gakusei dattara
• Negative: 行かなかった -> 行かなかったら ikanakattara

The た here is not a past tense. It marks the clause as completed before the main clause happens; the sentence as a whole can be about the future. Politeness is carried once, at the end: 雨が降ったら、行きません。

Step 2 - Know the four jobs たら does.

1. General condition (“if”). The main clause happens only if the condition holds: 安かったら、買います。 (Yasukattara, kaimasu.) If it’s cheap, I’ll buy it.
2. Temporal “when / after”, for a definite future event. If the event is certain to happen and you are only unsure when, たら is still the form: 家に帰ったら、電話します。 (Ie ni kaettara, denwa shimasu.) When I get home, I’ll call. You will definitely get home - so English says “when”, but Japanese still says たら. (Contrast 帰るとき, “at the time of going home”, from lesson 23: 帰ったら is strictly after arriving.)
3. Counterfactual (“if I had…”). With a state verb or adjective, たら happily describes a situation that is not true: お金があったら、世界を旅行したいです。 (Okane ga attara, sekai o ryokō shitai desu.) If I had money, I’d like to travel the world.
4. Discovery in the past. When the main clause is past, たら reports what you found on doing something - typically something unexpected, outside your control: 銀行へ行ったら、閉まっていました。 (Ginkō e ittara, shimatte imashita.) When I went to the bank, it was closed. Here nothing is conditional at all; it is pure narrative.

もし (moshi, “if”) is an optional early signal that a condition is coming: もし雨が降ったら、中止です。 It never replaces たら - the ら still does the grammar - and it is normal to leave it out. Use it when the condition is genuinely hypothetical, not for a certain future event (so not with 家に帰ったら).

Step 3 - 〜たらどうですか offers advice. Take たら and end the sentence with どうですか: 少し休んだらどうですか。 (Sukoshi yasundara dō desu ka.) Why don’t you take a short rest? Literally “how would it be if you rested?” - a soft suggestion, far gentler than a command.

Step 4 - Build と: non-past plain form + と. と attaches to the dictionary form or the ない-form, and never to a past form, no matter what tense the sentence has. 押す + と -> 押すと; 押さない + と -> 押さないと. Nouns and な-adjectives keep their だ: 学生だと, 静かだと.

Step 5 - と means “inevitably”. The main clause is presented as an automatic, invariable consequence - nobody chooses it:

• Natural law: 春になると、桜が咲きます。 Spring arrives; the blossom follows. No decision involved.
• Machines and mechanisms: このボタンを押すと、切符が出ます。 That is what the machine does, every time.
• Directions: まっすぐ行くと、右に駅があります。 The station does not appear because you chose it; it is simply there.
• Habitual repetition: 毎朝コーヒーを飲むと、目が覚めます。 X happens, Y always follows.

Step 6 - The hard restriction on と. Because と says the result follows automatically, the main clause cannot contain a human decision. と is blocked before:

• requests - ×〜と、電話してください
• invitations and suggestions - ×〜と、行きましょう
• commands - ×〜と、待て
• permission - ×〜と、帰ってもいいです
• intention and desire - ×〜と、買いたいです ／ ×〜と、買うつもりです

In every one of those sentences, swap と for たら and it becomes correct. たら has no such restriction - which is why, if you are unsure, たら is the safe default.

One thing と can do is stand in a past sentence, when the main clause is an unwilled discovery: 窓を開けると、猫が入ってきました。 (Mado o akeru to, neko ga haitte kimashita.) The と-clause stays non-past (開ける), and only the main clause carries the past tense.

Step 7 - This と is not the quotation と. 学生だと思います (“I think he’s a student”) is a completely different と - see lesson 25. Conditional と joins two clauses; quotation と marks what was said or thought, and is followed by verbs like 言う and 思う.

Two more conditionals, 〜ば and 〜なら, complete the system at B1. The full four-way comparison - which one to pick in which sentence - is taught in lesson 51; with たら and と you can already say everything you need at this level.

Forms & patterns

Making たら - it is only the plain past plus ら.

Making と - non-past only, never past.

Deciding between them.

Examples

Common mistakes

• 駅に着くと、電話してください。 (Eki ni tsuku to, denwa shite kudasai.) -> 駅に着いたら、電話してください。 (Eki ni tsuitara, denwa shite kudasai.) - a request cannot follow と. Anything the listener decides to do needs たら.
• 日本へ行くと、写真をたくさん撮りたいです。 (Nihon e iku to, shashin o takusan toritai desu.) -> 日本へ行ったら、写真をたくさん撮りたいです。 (Nihon e ittara, shashin o takusan toritai desu.) - 〜たい is a wish, so the automatic-result と is blocked.
• 高いたら、買いません。 (Takai tara, kaimasen.) -> 高かったら、買いません。 (Takakattara, kaimasen.) - たら is built on the past form: an い-adjective must first become 高かった.
• 学生たら、安くなります。 (Gakusei tara, yasuku narimasu.) -> 学生だったら、安くなります。 (Gakusei dattara, yasuku narimasu.) - nouns and な-adjectives go through the copula: だった + ら.
• 窓を開けたと、猫が入ってきました。 (Mado o aketa to, neko ga haitte kimashita.) -> 窓を開けると、猫が入ってきました。 (Mado o akeru to, neko ga haitte kimashita.) - と never attaches to a past form. Only the main clause shows the tense.

Tips

• When in doubt, use たら. It reaches almost everywhere と does, plus everywhere と cannot go. The one place it does not fully cover is the invariable general truth - natural law, mechanism, habit - where spoken Japanese will accept たら but と is the standard and distinctly more natural choice. Using と where たら is required, on the other hand, is simply ungrammatical.
• Test the main clause for a human decision. Ask “did somebody choose this outcome?” If yes - a request, a plan, a wish, an order - the door is shut on と. If the outcome just happens, と is available and usually the more natural choice.
• Do not trust the English “when” and “if”. English “when I get home” and “if it rains” both come out as たら; and Japanese と often translates as “when” even though it is a conditional. Decide by the Japanese criteria (automatic vs chosen, non-past vs past), not by which English word you started from.
• 〜ないと is worth memorising as a chunk. 眼鏡をかけないと、見えません - “if I don’t…, then…”. In casual speech the main clause is often dropped entirely, leaving 早く行かないと…… (“I’d better hurry”), which links straight into the obligation patterns of lesson 35.