"I was writing that letter all day."
Translation:Psal jsem ten dopis celý den.
You can't start a past-tense sentence with "jsem" or any of the various present-tense forms of "být" (as I learned when I made the same mistake...) But in this sentence you can say either "Já jsem psal/a..." or "Psal/a jsem..."
As I understand it, the "subject" personal pronouns generally can be omitted unless they're needed or wanted for clarity.
I will offer two GUESSES, neither of which may be correct...
It's often said that the part of the sentence that you want to emphasize is placed at the end. In this case, the important part could be that you spent ALL DAY writing that letter. So if "Cely den ten dopis jsem psal" was not accepted, that MAY be a reason why.
This is the scary one... "Cely den ten dopis jsem psal" sounds weird. Given that I'm learning too, that in itself is weird... unless some of this word order stuff that's been making me crazy is starting to sink in and I don't even realize it.
It does not really work as a normal sentence with normal pronunciation. Czech is a SVO language, I don't know why so many people insist on the verb in the final position, it isnot too common there. It would be there if you were cntrasting this verb with some other verb, but which one? I did not do ... with the letter all day, I was WRITING it all day. What would ... be?
The swntence could acually work as a form of reproach or reminder how long it took. In that case you can't use normal pronunciation, you have to strongly stress Celý den.