Demain, il sera sérieux.
Demaindemain
You would say "nous verrons demain" in French to mean "we will see tomorrow."
You can say "Ils sont sérieux" in French to mean "they are serious."
You can say "Je le verrai demain" in French.
avec moi
Et demain.
Après-demain.
"... part demain"
a demain
belle demain
Quand demain
You can say "Je te le dirai demain" in French, which translates to "I will tell you tomorrow."