You’d need to add some additional information for these two sentences to be understood, in the given context: That is the lawyer. (although there are two lawyer in this trial) He is the student. (although there are a lot of students in this class) As there is another lawyer/more students, you need to make it clear who you are referring to by providing more information, otherwise it is as if the other lawyer/students don’t exist. You could say for example - ‘that is the defendant’s lawyer’ or ‘that is the one of the lawyers involved in the trial’ or ‘that is the lawyer I told you about’, and ‘he is one of the students in the class’ ‘he is the student who got the best grade’, and so on. The idea is that for the sentence to make sense you need to identify that you are speaking about that lawyer or that student in particular by adding appropriate information.

no grammar it has been explained already. Example sentences. "they are the two lawyers" for more information "they are the two lawyers, one for the prosecution and one for the defence" "They are the lawyers for the plaintiff and the defendant" "that is your lawyer" "that is our lawyer" "that is their lawyer" "that is the other sides lawyer" (not the one defending you nor prosecuting you as the case may be)

2 are wrong: He is a lawyer, and He is a student. You can't say 'the' if there are more than one, unless you specify it: he is the defendant's lawyer/ he's the best student. :)

1 He is the defendant. - Yes 2 That is the plaintiff. - Yes 3 That is the lawyer. (although there are two lawyers in this trial) No. "The" has to refer to one specific person. You would have to make the sentence specific by saying, for example "That is the defence lawyer" or "That is the lawyer who's defending x". This distinguishes him or her from the other lawyer. Otherwise, you'd say 'He's one of the lawyers'. 4 He is the judge. - Yes And will the hearer understand that: 1 He is the defendant in this trial. Yes - if there's only one defendant 2 That is the plaintiff in this trial. Yes - if there's only one plaintiff 3 That is the lawyer in this trial. Not if there are two lawyers. If you say "That is the lawyer", this would mean that there was only ONE lawyer in the trial. If there are clearly two lawyers, the listener would be confused to hear you say "That is the lawyer". 4 He is the judge in this trial. Yes - it's normal to have one judge. The same goes for the student scenario. If you pointed to one student and said 'He's the student', we'd need to know why you are singling him out from all the other students. For example 'He's the student who cheated on his exam paper'. Unless you do that, he is merely A STUDENT - one of the many students in the class.

Yes. You can say all those things and you will be understood as you have described it. Good work!