You have already learned たら tara sentence in lesson 27 and 28. In those two lessons, with the following sentence pattern...
|Sentence 1||たら、||Sentence 2|
...basically you learned about:
When (where the moment is definitely happening in the future)
In this lesson, you will be learning yet another function of たら tara sentence - Discovery. The sentence pattern is similar, except that Sentence 2 is always in past tense.
|Sentence 1||たら、||Sentence 2 (Past tense)||- Express fact in the past|
The conjugation for Sentence 1 is the same as before, only with the difference that Sentence 2 is in the past tense. You might recall that Sentence 2 is always in present tense in lesson 27 and 28.
This expression indicates that Sentence 2 is realized or noticed when Sentence 1 is realized. Sentence 1 is always an action and Sentence 2 must be a state. Sentence 2 cannot be an action.
Basically you'll use the above sentence pattern in two cases.
Let's look at one example...
In the above example, both events have already happened.
In the past, I opened the curtain and discovered that it was raining
The dog has already been in front of the door before I opened it. But only when I opened the door, then I discovered that the big dog was there.
The department store was closed today. But I didn't know about that. Only when I went there, then I found out that it was closed.
The lesson had started before I arrived at school. I only discovered that when I reached there.
The letter from my friend arrived before I opened the letter box.
Both events had already happened in the past. That means, in the past, I ate ice-cream, then I had a stomach-ache. If I didn't eat ice-cream, I wouldn't have a stomach-ache.
Similarly, for the rest of examples, something happened unexpectedly as a result of the actions in Sentence 1.
In the first case of たら tara Sentence for Discovery, it normally delivers a surprise feeling. You discovered something and become surprised. In the second case, what happened next (as a result of the first action) was unexpected.
In either case, the most difficult part is the regulation you need to take note of when using this sentence pattern.
This regulation is quite self-explanatory because if you have speaker's intentional action in Sentence 2, it's no longer a discovery, nor it's a surprised or unexpected event.
Let's see an example...
Both events in the above example are facts in the past. I went to Japan last year and I stayed in a hotel.
However, you cannot connect the two sentences with たら tara sentence. Because the action of staying in hotel was an intentional act decided by you. You chose to stay in a hotel. This is not a surprised or unexpected event.
You cannot say...
But you can say...
The above example was just two actions in order of occurrence using て-form. First I went to Japan, then I stayed in a hotel.
Therefore, for たら tara Sentence for Discovery, no intentional acts by the speaker in Sentence 2.
Lesson 27: たら (tara) Sentence.
Lesson 28: たら (tara) Sentence Special Case.