The weather was gloomy, but the atmosphere was cheerful. (Complex)

Compound Sentence: The weather was gloomy, but the atmosphere was cheerful. 
Complex Sentence: Though the weather was gloomy, the atmosphere was cheerful.

→ Compound Sentence এ but থাকলে Complex Sentence এর ক্ষেত্রে Though/Although ব্যবহৃত হয়।

• Complex Sentence: 
→ যে sentence- এ একটি Principal Clause এবং এক বা একাধিক Subordinate Clause থাকে, তাকে Complex sentence বলে। 
→ Complex Sentence এর Subordinate Clause এর শুরুতে সাধারণত if, though, although, as, because, since, so that, that, until, till, unless, when, why, who, which, where, how, before, after, whether, while বসে।

More Example:
Compound sentence: He is poor but he is honest
Complex Sentence: Though he is poor, he is honest.