sad
The French word 'triste' is used in much the same way as 'sad' in English, as an adjective to describe someone's feelings when they are not happy, or something that makes you feel this way. For example, 'Je suis triste' means 'I am sad'.
He is sad today.
In this sentence, 'triste' is used to describe the emotional state of a person. Note that the adjective 'triste' follows the verb 'est', matching the English construction.
The movie was really sad.
Here 'triste' describes the emotional content of the movie, showing that it has the capacity to induce sadness. The verb was (était) is used here in the past tense implying that the speaker watched the movie in the past.
Do not be sad.
This is a command, telling someone not to be 'triste'. The verb 'soyez' is the imperative form and it is used here to give advice or orders.