"Dann dachte seine Mutter nach."
So 'nachdenken' means to think about/contemplate, and just 'denken' is used where there is a single thought?
Yes indeed. And so? How does one infer from this translation whether her thoughts were single or multiple ?
Is "dann . . . nach" a phrase that works together? I tried translating the two words separately, but was marked wrong. I feel like it's hard to tell when I need to literally hit every word in the sentence in my translation vs. when there's more of a natural translation.