"j'ai un sujet très sérieux"
You can say "Ils sont sérieux" in French to mean "they are serious."
Sérieux (if what you are talking about is masculine) or serieuse (if what you are talking about is feminine)
le thème (as in topic of essay/research) le sujet (as in topic of conversation)
'quel est mon sujet?'
To say "very close" in French, you would say "très proche."
Nous parlons serieux.
Je suis sérieux/sérieuse
Demain, il sera sérieux.
"Very" - tres
"Très grand" is how you would say "very big" in French.
In French, 'not very well' is pas très bien.
Very big is said: très grand in french.