لَيْسَ negates the state or description expressed by its khabar.
لَيْسَ زَيْدٌ مُجْتَهِدًا
Zayd is not hardworking.
- زَيْدٌ is the marfu ism of لَيْسَ.
- مُجْتَهِدًا is its mansub khabar.
A neutral practice example is لَيْسَ الطَّرِيقُ طَوِيلًا: the road is not long.