Confusion - Past Simple vs Past Continuous

[cnoko]

Hello all, This site only helps me to decide the other problems, but one remained currently. I have a confusion on grammar from Cambridge (in use 3th edition).

When Karren arrived, we were having dinner. (= we had already started before she arrived)
When Karren arrived, we had dinner. (= Karren arrived, and then we had dinner)

I can't understand why does that happen.
How do I find when is past, when is continuous?

Thanks in advance.

[tiorthan]

The continuous aspect (regardless of whether past or present or whatever) means that an action is "continuing" through a specific point or period of time. So what your first sentence really says is this:

When Karren arrived = this is a point in time which is our point of reference for this sentence
we were having dinner = the action of "we have dinner" happend in the past (this is encoded in the "were") and is continuing through our temporal point of reference.

Since it is continuing it must have started before and end after the action that describes the point in time.

Your second sentence doesn't have an additional aspect (sometimes said to have the "simple" aspect). Which means that there is no additional information about how time "flows" in the sentence. Usually, this means that the actions described occur in their natural order. So, since the first part still provides a temporal reference the second part is now interpreted to begin where the previous part left which is when Karren just had arrived.

[cnoko]

I think that I understand.
thank you very much.