Opposite adjectives:
1. logical - illogical
2. responsible - irresponsible
3. believable - unbelievable
4. possible - impossible
5. active - inactive
6. balanced - unbalanced
7. forgettable - unforgettable
Sentences:
1. His decision was illogical.
2. It's irresponsible to leave the door open.
3. Her story sounded unbelievable.
4. It is impossible to complete the task in 1 hour.
5. He is very inactive during the weekdays.
6. The box was unbalanced, and everything fell over.
7. This was an unforgettable experience!