Let me first explain 是……的 as a structure. It is a tricky way to talk about past events in Chinese. You are right in saying that you can use this structure to emphasise certain details about an action, such as where or when it happened. 是……的 is usually used to talk about past events (note, it can be used for present as well, so don't confuse this as something that can only be used in the past). Here is a general breakdown of the structure:
[subject] 是 [detail] [action] 的
Wǒmen shì zài Lúndūn rènshi de.
We met in London.
It was in London that we met.
Wǒ shì gēn wǒ gēgē yīqǐ kaichē qù Qīngdǎo de.
I drove to Qingdao with my brother.
It was with my brother that I drove to Qingdao.
Tāmen shì qùnián bān de jiā.
They moved house last year.
It was last year that they moved house.
Each of the example sentences for 是……的 has been given a double translation to show how this structure draws attention to certain details of the action.
Now lets look at 了. One of the uses of the particle 了 in Chinese is to mark completed aspect, which in English would be represented by the present perfect (past tense). Many learners of Chinese confuse this for 了 being about past events or the past tense. It's important to remember that this is not the case, it is simply an aspect marker that shows the changing state of something. Often times this involves the past tense, but 了 can equally be used in the present and future.
My advise, however ambiguous this may seem, is to not over think it too much. Read a lot, simple books or something you are familiar with (such as Harry Potter, the ebooks are relatively easy to find online), this way you will naturally come to understand its use. Unfortunately, this is probably the most questioned idiosyncrasy in Chinese and there is still not well-put-together answer I'm afraid.
Nonetheless, I hope this has helped and keep up the good work!
- Lin Ping