Learn Arabic (Modern Standard): لم تكن أختي النشيطة تسهر في الماضي. - My active sister did not use to stay up late in the past.

To negate a habitual or continuous action in the past (like "did not use to"), Arabic uses لم followed by the "to be" verb in the jussive case (here, تكن for a female subject), and then the present tense of the main verb (تسهر). It literally builds the structure "she was not staying up late."

Why is the verb تكن instead of تكون?

The negative particle لم forces the verb after it into the jussive case. For a hollow verb like تكون (she is), the final letter gets a sukuun (a marker showing there is no vowel). Because Arabic avoids having two unvoweled letters next to each other, the middle "waaw" is dropped to make it easier to pronounce, resulting in تكن.

Why does the adjective النشيطة (active) have the definite article ال when أختي (my sister) doesn't?

أختي is already definite because it has the possessive suffix ـي (my). Adjectives must match their nouns in definiteness. Since "my sister" is a specific, definite person, the adjective describing her must also be definite, which we do by adding ال to make النشيطة.

Why is the subject أختي النشيطة placed between the two verbs تكن and تسهر?