"Mtoto ni mwanafunzi."

Translation: The child is a student.

[amy777886]

How do you know "the" or "a"

[Rachel922386]

Context I guess.... "A" is an indefinite article and the statement "A child is a student" is an unreasonable generalisation unlikely to be used in practice.
"The" is a definite article and the statement "The child is a student" refers to a specific child and is the more logical statement