Hola Julie

In the first lesson, we're giving two ways to say "could" in the past, and that can be said with pude or he podido, i.e. using the preterite or the present perfect.  Then we continue to say that the nuances are different, depending on how the speaker sees the action; this could be referring to something that the speaker sees as disconnected from the present, using the preterite for this, or as an action where the moment it started or ended is not relevant. 

Later on we say that the use of these tenses doesn't mean that a specific time frame has to be used for these to be referring to one type of action or another; what we need to consider is that if the speaker wants to say that he did something and this action is considered by the speaker as a disconnected action from the present, the speaker will use the preterite, say for example: "Fui al cine"; on the other hand, if the speaker wants to say the same thing but still feels this is connected to the present or doesn't think it is important to express it as an action that had a beginning and an end, then the speaker will use the present perfect "He ido al cine". 

If we take one example from the lesson, for example:

Here there is no time frame mentioned, but the speaker is using the preterite thinking of this action as something that couldn't be done at some point in the past, a specific point, that now feels disconnected from the present; the speaker is probably thinking about the moment, yesterday, when he was trying to book the ticket for the concert, and he didn't manage, and that was the end of that action.
But he could have also said this:

Here, the speaker is not necessarily thinking about that precise moment and when the action of trying to get tickets finished. This is seen as a more general action that simply happened without emphasising the end of it. 
The uses of the preterite and the perfect tense in Spanish are quite tricky because that distinction doesn't exist in English, and the uses of the preterite and the perfect tense don't always coincide with the uses of these tenses in Spanish. It's a topic that need to be revisited many times. 

Saludos

Inma

Hola Julie

This other lesson is only about conjugation. It's only to learn how to conjugate poder in the preterite. For this reason we don't specify the nuance in each example, as it's all about learning the endings for this conjugation. 

In all examples that you see in that lesson we need to assume that the speakers are seeing these actions as actions that are already disconnected to the present tense, seen as actions that had an end. 

It is a lot easier to see the difference in these A2 lessons were there are time frames used and the actions is more easily seen as disconnected or still being part of the timeframe where the speaker is.

When to use the perfect tense vs the simple past

Preterite for completed actions

The hardest thing is to get this different nuance when timeframes are not used at all. 

As I said, this contrast of past tenses in Spanish is one of the topics that bring more questions and doubts from students so you are definitely not alone and it is not silly at all not to get it straight away : ))

Saludos cordiales

Inma