sad
The Italian word 'triste' is used to describe a state of unhappiness, just like in English. It can describe a person's mood, or to describe particular situations or events are causing sorrow. For example, 'Sono triste' means 'I am sad'.
It's a sad day.
This is a simple sentence, used to express a general feeling of sadness about the day. The adjective 'triste' is used here to describe a day, indicated by 'un giorno' in Italian.
He is very sad today.
This sentence is an expression of a person's current emotional state. The person identified as 'he', denoted by 'lui' in Italian is described as being 'very sad', translated as 'molto triste' in Italian.
This song is sad.
In this sentence, we're stating that a specific song, denoted by 'questa canzone' in Italian, is sad, a sentiment expressed by the Italian adjective 'triste'. This could be based on the melody, lyrics or overall tone of the song.