"His father was a cheerful man."
Translation:Его отец был весёлым человеком.
This should be accepted, too.
In Past tense, in «X is Y» sentences you can usually put Y in either nominative or instrumental case.
I personally use Instrumental in all such sentences, and don’t use Nominative here. However, Rosenthal’s style guide (here, in Russian, §179.3.1) recommends using Nominative for unchangeable things (e.g. name, nationality), and Instrumental for things that can change (e.g. occupation), so your form is actually in line with Rosental’s recommendations.
This sentence requires instrumental. Being a cheerful person is not a permanent state. Use instrumental with something that is no longer true. In this case, he was a cheerful person, but he isn't anymore - either because he is now a grumpy old man or because he is dead. So it must be instrumental because it is a changed past state.
Thank you for explaining! Спасибо большое за объяснение!
However, I do think name and nationality are changeable. Theoretically, I can change my name to Igor’ Vasil’jevič Kurčatov and move to Russian any day now and, after a while, get a Russian passport. Thus, both my nationality and my name would be changed. On the other hand, what is not changeable?
I have used a wrong word when I said 'nationality'. :D In Russian, «национальность» is closer to English 'ethnicity, ethnic background', and 'nationality' is «гражданство». I was thinking about национальность and not about 'nationality' when I was talking about unchangeable things.
would it be like the diference between "ser" and "estar" in romance languages? E.g. "sou feliz/estou feliz"?
Technically, yes. However, I personally don't follow this rule in my speech.
If the sentence were in present tense, we would have used nominative, right? "Его отец - весёлый человек". How come we have to use instrumental because it's in the past tense?
If the sentence were in present tense, we would have used nominative, right? "Его отец - весёлый человек".
In fact, nominative is also possible in the past tense («Его отец был весёлый человек» — I personally don’t speak like this, I use Instrumental in the past tense, but Nominative+past is not uncommon in books and maybe in other regions).
How come we have to use instrumental because it's in the past tense?
I guess this is an exception that needs to be memorized. There is little logics in this.
Usually, verbs don’t change the cases depending on the tense. But «быть» is a highly irregular verb (it’s present tense есть — when it’s not omitted, of course, — doesn’t even look remotely similar it’s past tense был!), and, well, it does use Instrumental in past and future tenses, but not in present.
I guess it’s because Instrumental is often used for transformations (ма́сло ста́ло мя́гким ‘the butter became soft’, кня́зь оберну́лся во́лком ‘the prince turned into a wolf’), and using the past and the future tense of ‘to be’ implies some transformation compared to the Present Tense.