と～た・
Telling what happened: you did something, and there it was. The second half is what you found, or what happened then. In conversation it is mostly たら; と is how stories tell it.
たら: in conversation
When I went into the shop, someone I knew was behind the till.
と: in a story
When I opened the drawer, there were some old letters at the back.
たら
Verb in the た-form + ら、… た
開けたら. The sentence ends in the past, often in ていた.
と
Verb in the dictionary form + と、… た
開けると. No た before と, though it all happened in the past.
What you found, what happened
The first half is what you did or where you got to: you opened it, went in, arrived, woke up. The second half is what met you there. It often ends in ていた, because it was already so before you came.
When I got to the venue, registration had already closed.
When I got home, there was a parcel outside the front door.
When I took the lid off the box, a small insect flew out.
Not already there: this happened at that moment.
Not the next thing you chose to do
English "when I went into the shop, I bought some bread" is two things you chose to do, one after the other. About the past, たら can't say that. The て-form does.
Not said: 店に
(When I went into the shop, I bought some bread.)
When I went into the shop, I bought some bread.
と or たら?
When I woke up in the morning, everything outside was covered in snow.
How you would tell a friend.
When I woke up in the morning, everything outside was covered in snow.
How a story or an essay tells it.
The meaning is the same.
Things people say
てみたらwhen I tried it
When I tried it, it was better than I expected.
気がついたらbefore I knew it
Before I knew it, it was morning.
よく見たらon a closer look
When I looked properly, it was someone I didn't know.
Close neighbours
たとたん（に）N3
In that very instant, and it startled you.
The moment I got to the station, the train pulled out.
たところN2
The same in reports and polite accounts: you checked, asked or tried, and this is what came of it.
When I tried a different route, I arrived ten minutes earlier than usual.
たらN4
About what hasn't happened yet, the same たら is "when" or "if", and a request or a plan can follow.
Please call when you arrive at the station.
とN4
With いつも or よく, the same と… た is a habit in the past: "whenever".
Whenever my grandmother looked at old photos, she would always tell the same story.
Check yourself
1Which sentence sounds natural?
Choose an answer to see why.
2Which one fits?
窓を
Choose an answer to see why.
3What does this mean?
駅に
Choose an answer to see why.
More detail
In novels, と also joins two things a person did, the second straight after the first. たら can't.
The man stood up and opened the window.
More sentences
I walked into the room thinking nobody was there, and found a senior colleague waiting.
When I opened the email, it said that I had passed the interview.
When I switched on the machine, an alarm suddenly started sounding.