I find myself troubled by the direct link between "Я не должен был учиться вчера" and "No debí estudiar ayer". If memory serves, the Russian sentence expresses the absence of obligation to study, and the Spanish one appears to be a statement of regret at having studied (such as despite an obligation not to study).

The "didn't need to" vs "needn't have done" distinction?
Rather blurry in Russian and often apparently non-existent, given that we can use the same modal должен (должна, должно) to mean things considered different in other languages such as English.
In fact, while this sentence could mean 'I didn't need to study yesterday", in my mind I find it to fit "I needn't have studied yesterday" better. In fact, it could also mean a negative obligation (I wasn't to study yesterday), hence the unwillingness to use it in the 'absence of obligation' sense, since this could end up confusing.

If I meant to express the absence of obligation, I would more likely say Мне не нужно было вчера учиться, and funnily this other sentence could also express regret, standing for both "I didn't need to" and I needn't have done".
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The most curious thing, however, is that I don't see any link currently despite it being clearly shown in the logs. What's wrong?