"He is the judge."
Translation:Er ist der Richter.
There is a difference between "He is a judge" and "He is the judge".
"He is a judge" describes the profession of the man. In German, this is expressed as "Er ist Richter." The English sentence uses an indefinite article, but the German sentence uses no article.
"He is the judge" describes something else that is more context-dependent. For example, it could mean this man is the judge of a particular court or case that is being discussed. In any case, this sentence is expressed as "Er ist der Richter" in German. Each sentence uses a definite article.