"A tomato is tasty."
Translation:Pomidor jest smaczny.
1) "Pomidor" is masculine, so it cannot use a neuter form.
2) The "X to Y" construction works only when both X and Y are noun phrases. "A tomato is a tasty fruit/vegetable" could be translated both as "Pomidor jest smacznym owocem/warzywem" and "Pomidor to smaczny owoc/smaczne warzywo".
There is another excercise where we are asked to translate "pomidor jest smaczny" (approximatly). What is the difference?