"Since it was raining, they stayed indoors and played board games." This is a -

• "Since it was raining, they stayed indoors and played board games."
→ This is a - Complex Sentence.
→ বাক্যটির Subordinate Clause- এর শুরুতে so that থাকায় বাক্যটি Complex Sentence.

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

More Examples of Complex Sentences:
- Although it was raining, they decided to go for a walk in the park.
- Because she had missed the bus, she had to call a cab to get to work.
- After the movie ended, we went to a nearby cafe for some coffee.
- Since they lived far away, they only visited their grandparents during the holidays.

Source: Advanced Learners Communicative English Grammar & Composition By Chowdhury & Hossain.