Do not translate bare تَفْعَلُ without context.
● هِيَ تَكْتُبُ — She writes. Here تَفْعَلُ is third-person feminine singular.
● أَنْتَ تَكْتُبُ — You (one man) write. Here the same pattern is second-person masculine singular.
The letters are the same, but the stated pronoun changes the grammatical slot and the translation.