Guys I need help

For school I had to translate: “The danger was serious”, “You are brave”, and “On account of the common salvation of men, Christ came into the world.” The answers I got were “Periculum gravum erat”, “Fortis es”, and “Propter salus communis homini, christus in mundum venit”. The answer key said that the correct answer is “periculum erat grave”, “estis fortes”, and “propter salutem communem hominum christus in mundum venit”. Why? Is there something consistent in doing wrong? Specifically on my translation to “fortis es” they said it was estis but my flashcards say that es is the translation for “you are”.

(After a bit of desperately trying to work out the first sentance I found in one of my books that 3rd declension adjectives have 3 ways to decline them and that the correct answer was grave and that I was declining it like 2nd declension adjectives, but I don't think that explains the other problems.)