Arabic Past Tense Question.?

I've got a question on Arabic past tense verbs. Apparently, to form the past tense of a verb in Arabic, you would extract the three consonants, (for instance, ktb in the verb kitab) and place them in order, and add your vowel marks. Then you would add "tu" at the end. So it would look like "katabtu", right? But in most Arabic writing, the vowel markings are left out. So the word for "I wrote" would look like "ktbt", but the words for "you wrote" and "she wrote" would look the same because the vowel marks are left out. So how could you tell if "I wrote" or "You wrote" is being said?