So asks a person who must know word definitions are true, and the structure of language to form sentences is true and useful. And knows what a question is and asks them. So how do you know all this?
That depends on what true is and what useful is.
The problem is that there are at least 5 different versions of true, if not more. So which one is true and how do you know that the other ones are false.
As for useful, that has no objective referent or method for true as objective. So what in some cases can be useful to you, is useless to me and so in reverse.
We are no debating if a part of the everyday world is in effect and in practice objective. We are debating if from that follows, that all what is true and useful are objective. That is not the same.
It appears that you for some forms of subjective are unable to understand that you are subjective and that in some cases true and useful, is what is true and useful to you, but not me.
The falsifacition of whether something is objective or subjective, is in one version if it can be done differently subjectively. And that is the case here, since we in effect disagree subjectively.