"Dad is not a child."
Translation:Baba si mtoto.
"Siyo." can be used as "no" (It is not so.)
"Ni" is the copula (standard form of the conjugated verb "to be"); "si" is it's negative. (Basically you use these two whenever equating two things to one another.) Mimi ni mwalimu(. Mimi) si mwanafunzi. (I am a teacher, (I am) not a student.)