Translation:Its head could not have been bigger than an apple.
My translation: "His head could not be bigger than an apple."
Translation: Its head could not have been bigger than an apple.
Why can't "jeho" be translated as "his"? What does "jeho" refer to in this sentence? If "nemohla být" means "could not have been", how would you say "could not be"?
We accept both ITS and HIS. It could have been a marionette or some animal we are talking about here, which in English would be IT, while in Czech it could be anything. The Czech sentence suggests that the 'something' is masculine in Czech but it might not be the case in English. But both ITS and HIS are among accepted translations.
Could not be = nemůže být
Very interesting! There is a definite difference between "he couldn't be there" and "he couldn't have been there" in English, but apparently not in Czech. "He couldn't be there." because he had some other obligation or was ill, for example, at the time of the event, which prevented him from being there. "He couldn't have been there." As in a court case, the evidence shows that it was impossible for him to have been there because he was somewhere else, such as taking a class or at a party, where there were witnesses to prove where he was, so he is innocent of the crime, for example. I hope this is not too confusing, but for American English speakers, the difference should be clear.
But note that "could be" has two meanings in English - the first is the one you described, the past tense meaning, which translates to Czech as "mohlo být" (same as "could have been") - but the other meaning is conditional ("it could be fun to try this") which translates as "mohlo by být" (using the conditional). So here it's the other way around - English combines two meanings into one expression, from the Czech perspective.
Protože to nedává smysl. S "couldn't be" by to znamenalo, že jeho hlava nemohla být větší proto, že tomu něco bránilo - že ta hlava nedokázala dostatečně vyrůst. Taková věta by šla použít snad jen v nějakém hororu o šíleném vědci, který pěstuje těla v umělých líhních. Druhý možný překlad toho "couldn't be" by byl "jeho hlava by nemohla být větší"... s úplně stejným omezením použítí na jakýsi obskurní horor.
S "couldn't have been" to znamená v podstatě, že "jeho hlava určitě nebyla větší..." - což je úplně normálně použitelné.