the adjective which describes something is following the described word in defining or undefining , masculine or feminine , singular or plural
So, we put ال in front of the objective when the described noun is definite
eg: قابلت الرجل الكبير
قابلت رجلا كبيرا
قابلت رجالا كبارا
قابلت الرجال الكبار
I hope that I could help you,
good luck with your lessons
Best wishes