"Йому було сумно вчора."

Translation: He was sad yesterday.

[zchale6]

Why is the phrase in this form, and not as "Він був сумний вчора"

[buddhist_monkey]

Your translation is also correct

[steampunkgiraffe]

what case is Йому part of?

[jdc2179]

I believe it's dative here.

[nadine29]

Why було?

[jdc2179]

I think a little translation would be something like "It was sad to/for him yesterday." So, було refers to 'it.' However, the meaning translates to "He was sad yesterday," which is the source of confusion, since the subject changes between the literal and figurative translations.