Question about using たら in the past tense

So I've recently finished Genki II and ordered the Tobira kanji and textbook. The kanji book arrived first so I flipped open the review exercises and ran into something I don't think I'm familiar with.

Here is the sentence:
カフェテリアに食事に行ったら、クラスメートが楽しそうに音楽を聞きながら、ご飯をたべていた。

I'm wondering how to understand the たら when the sentence is in the past tense.

Also, does anybody know if there are translations for the Tobira exercises?

Thanks

Question about using たら in the past tense

It is a "when" statement: "when A, B" and I think it carries a nuance of B having been either unexpected or perhaps undesired....something we would convey in English through intonation rather than a certain construction.

Unless you're studying to be a translator, try to get along without translations.

Ah, alright. I was thinking about it just being used as 'if' which was really throwing me off.

Thanks.

Question about using たら in the past tense

"If" and "when" sort of bleed into each other a bit, don't they?

That's a prime example of how a translation habit can trip one up, though. Having the word "if" chained to 〜たら in your mind may have helped when first learning, but hanging onto such inflexible associations can hold you back later, as you have seen for yourself here.