the lady was really serious
John is a very serious man This is a very serious matter
"thoughtful not joking" is not a sentence and it does not contain the word "serious". Serious is an adjective, a word that describes a noun. Example: Justin has a serious expression on his face. The adjective "serious" describes the noun "expression".
"Are you serious?"
"They" is the subject of that sentence.
The judge's presence lent a more serious tone to our dinner group. He was serious yesterday.
"this play i serious so act like you care"
There are many people who are serious about school, while others are not.
'I want to be in a serious relationship.'
The sentence should be punctuated as: "The district attorney you're serious about has been arrested."
He's so jocular that you can't get any serious sentence from him.
I had the most serious look on my face when my mom said to go clean my room.
the judge explained that because his offence was so serious,his prison sentence was protracted.