the lady was really serious
John is a very serious man This is a very serious matter
"They" is the subject of that sentence.
"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?"
Serious answer: Wait as the car draws near. Non-serious answer: This sentence will end with the word 'near'!
The judge's presence lent a more serious tone to our dinner group. He was serious yesterday.
There are many people who are serious about school, while others are not.
"this play i serious so act like you care"
'I want to be in a serious relationship.'
He's so jocular that you can't get any serious sentence from him.
bacillus is very serious
Fraud is a serious criminal offence.