We use Past Simple with actions that started and stopped in the past, i. e. they are completed:
I broke my leg
This is the main difference between Past Simple and Continuous Line of Past Tenses
She was doing her lessons (It’s unclear now if she did her lessons or not)
She did her lessons (it’s clear now that she finished doing her lessons)
They were going to the movies when we met them yesterday (it’s unclear if they went to the movies)
They went to the movies yesterday (we know that they went to the movies…. )
You don’t really have to read the following unless you want to:
Some teachers explain other tenses like Present Perfect from the point of view of representing the fact of an action. I avoid this terminology as Past Simple, in the example above, shows the fact of going to the cinema as well. Though explaining Present Perfect from this point of view has some grounds if explaining like so: Present Perfect is used to show the fact that something has happened and Past Simple is used to show when or how something happened. For the purpose of this lesson let’s just skip this controversial discussions here)
They were going to the movies when we met them yesterday. After we told them that the movie was not good, they decided not to go there. Or: they knew that the movie was not the best but still they went to see it.
See how different the outcome can be? That is why it is unclear if the action was complete or not.
We know that the action was complete from this sentence: they went to the movies.
If we use the negative form of this tense, it is still a completed action that just did not take place, i.e. the action didn’t start at all:
They did not go to the movies.
In a nutshell:
We use Present Simple when the actions are complete, actions that started and stopped in the past, in the negative constructions, we can revert this as: a completed action did not occur (start) at all. Simply saying, an action started and stopped or did not start at all.