"Fall is not a good period."
Translation:Jesień to nie jest dobry okres.
I am not sure what you mean.
"Jesień jest dobrym okresem" and "Jesień nie jest dobrym okresem" use the instrumental case.
"Jesień to dobry okres", "Jesień to jest dobry okres", "Jesień to niedobry okres" and "Jesień to nie jest dobry okres" use the nominative case.
If the difference between the cases is the problem, then remember that when you use "to" or "to jest", you need to put the nominative form, and when you use "jest" - the instrumental one. And if the problem is:
"Widzę dom" (N.) but "Nie widzę domu" (G.)
then you have to know that not all negations use genetive:
"To jest dom" but "To nie jest dom", however "Tu jest dom" and "Tu nie ma domu".
Remember, "to" asks for nominative.
Actually, it is difficult to say which one is more popular. All of them sound equally natural in some sentences. I checked The National Polish Corpus with a sample sentence and this is what it shows:
"on jest moim przyjacielem" - 4 results
"to jest mój przyjaciel" - 10 results
"to mój przyjaciel" - 20 results
As you see, the shortest version is the most common (probably because of economic values). I would say that "jest" + [instrumental] is the least universal construction. When you want to say "It's the end", you cannot say "To jest końcem", because you stress "to", you mean that particular "to". Therefore you can say "To miejsce jest końcem (naszej podróży)" (This place is the end of our journey). And maybe that's the distinction: "to jest + [ins.] is more like "this (thing) is" while "to/to jest + [nom.] is more like "it is".
"To mój pies" - "It's my dog"
"To jest moim psem" - sounds like "That something/the thing is my dog". You don't use this constructions with beings. "To jest moim wujkiem" -> "That thing (an alien maybe) is my uncle". However you can say "To zwierzę jest moim psem" or "Ten człowiek jest moim wujkiem".
Ok if we use "to"
However, if we don't use "to", then: "Jesień nie jest dobrym okresem" (instrumental) is correct "Jenień nie jest dobrego okresu" (genitive) is incorrect. Does someone know why ?
And which form do you usually use in polish ? With "to" + nominative ? Or without "to" + instrumental ?
Well, a version without 'jest' is accepted, but I'd really recommend using it. Without it, you arrive at "Jesień to nie dobry okres". It can be read as a misspelling of "niedobry", an adjective which means something like not-good, bad-ish. So 'jest' makes it clear what you meant.
You have two noun phrases: "jesień" and "dobry okres". If you decided to use the "X to Y" construction... you cannot leave out "to", this doesn't make sense. You can omit 'jest' in such a construction and in fact you usally do.
But as here that could be understood as a 'niedobry' (which would change the meaning a bit, although it's hard to precise it), better leave the 'jest' as well: Jesień to nie jest dobry okres.
If you decide to use "X + a form of 'to be' + Y", then you have to put Y in Instrumental: "Jesień nie jest dobrym okresem".