Even a baby`s first impression leaves a definite trace on its brain.
Нужно поставить глагол-сказуемое во все временные формы Indefinite в страдательном залоге(Present,Past,Future),т.е. только в 3 формы

A definite trace is left on a baby's brain even by its first impression.
A definite trace was left on a baby's brain even by its first impression.
A definite trace will be left on a baby's brain even by its first impression.