A فعل مضارع expresses an action or state in the present or future. Arabic uses the same verb form for both times; the context tells us whether “now” or “later” is intended.
Present: هُوَ يَكْتُبُ الدَّرْسَ — He is writing the lesson.
Future by context: the same form can mean “He will write the lesson” when a future context is supplied.
Do not decide the time from the verb form alone; read the surrounding words and situation.