"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".