Here are some common words along with their C1 synonyms:

1. Happy (Delighted, Ecstatic, Contented)
2. Sad (Melancholy, Despondent, Disheartened)
3. Big (Enormous, Immense, Gigantic)
4. Small (Tiny, Petite, Minuscule)
5. Good (Excellent, Superb, Admirable)
6. Bad (Terrible, Awful, Atrocious)
7. Beautiful (Stunning, Gorgeous, Exquisite)
8. Ugly (Hideous, Repulsive, Unattractive)
9. Tired (Exhausted, Fatigued, Weary)
10. Smart (Intelligent, Clever, Brilliant)
11. Stupid (Idiotic, Foolish, Imbecilic)
12. Interesting (Fascinating, Engaging, Captivating)
13. Boring (Tedious, Monotonous, Dull)
14. Difficult (Challenging, Arduous, Demanding)
15. Easy (Simple, Effortless, Straightforward)
16. Confident (Assured, Self-assured, Poised)
17. Nervous (Anxious, Uneasy, Apprehensive)
18. Brave (Courageous, Fearless, Valiant)
19. Cowardly (Timid, Craven, Fainthearted)
20. Successful (Accomplished, Prosperous, Triumphant)