"That is the good teacher!"
Translation:Das ist der gute Lehrer!
The preceding article is "der" (a definite article/"der" word) and it is in the nominative case. Therefor the ending added to "gut" is -e. See this chart for "weak inflection": http://en.wikipedia.org/wiki/German_declension#Weak_inflection.5B6.5D.5B7.5D
But the article (der/die) must match the gender of the teacher (Lehrer/Lehrerin)