The chapter is specifically about the final feminine tāʾ of the perfect verb. The initial t of taktubu is a present-tense prefix, not this feminine past-tense suffix.
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Why the Chapter Specifies Past Tense
Feminine Tāʾ in the Past Tense: Form and Agreement
The chapter is specifically about the final feminine tāʾ of the perfect verb. The initial t of taktubu is a present-tense prefix, not this feminine past-tense suffix.