"Arabic exercise (spelling and verbs)

verb: to write (root: KTB: كثب)
Past tense: he wrote: كَثَبَ
Presnt tense: he writes:يَكْتُبُ (ya+ u-ending)
Future tense: he will write:سَيَكْتُبُ (s+ya + u-ending)"

You wrote ث instead of ت

Past Tense - الـمَـاضِي

كَـتَـبَ - Third person masculine singular

Present Tense الـمُـضَـارِعُ

يَـكْـتُـبُ - Third person masculine singular

Imperative الأَمْـر

اُكْـتُـبْ - Third person masculine singular

Future Tense الـمٌـسْـتَـقْـبَـل

سَـيَـكْـتُـبُ - Third person masculine singular

سَـوْفَ يَـكْـتُـبُ - Third person masculine singular

"Therefore:
Past: root + a-ending
Present: ya + root + u-ending
Future: sa+ya + root + u-ending"

This applies for third person masculine singular only.