# Why is it 日本語がわかります instead of 日本語をわかります?

From what I understand, は is the topic marker, が is the subject marker, and を is the object marker. One of the first sentences I learned doesn't seem to fit the rules I described above. The sentence is: > わたしは日本語がわかります. watashi wa　nihongo　ga　　 wakarimasu I　　　topic Japanese subject understand 'I understand Japanese.' In this sentence, why does 日本語 have the subject marker attached to it? Shouldn't 日本語 be the object of the sentence?