Questions & Answers about La carta no es triste.
Why isn't it trista to match the feminine word carta?
Adjectives that end in -e, like triste, do not change for gender. You use the exact same word for masculine nouns and feminine nouns. Since it doesn't change, triste works perfectly for the feminine noun la carta.
If 'triste' is a feeling, why do we use 'es' instead of 'está'?
Why does no come before the verb es?
In Spanish, the negative word no is always placed directly before the verb. While in English you say "is not", Spanish builds it literally as "not is" (no es).