"This is not a good period."
Translation:To nie jest dobry okres.
Only in masculine personal plural. Check out this page: https://en.wikibooks.org/wiki/Polish/Adjectives - the case I've mentioned is the only one marked red in this table and as it says there: "final consonant group must be palatized (or "softened"), and "i" or "y" must be added, depending on result". They also have some examples for softening rules.
In other words, "ry" will convert to "rzy", if you can complete the sentence: "Oni są...", for example - "Oni są dobrzy", "Oni są mądrzy", "Oni są starzy".
No, not here. I think this sentence is like "This is not a good period of my life", something longer. While "To nie jest dobra pora" is like "This is not a good time, I am very busy at the moment".
Can it be "To jest nie dobry okres" ? Does it really matter if "nie" should be before "jest"?
It matters. You should negate "being a good period" and not the adjective "good". If you negate the adjective, 'nie' gets glued to it, creating "niedobry". Which is not an exact antonym of "good" it's more like "not-good" than "bad". Anyway, it's not a translation.