Having attended the meeting, they returned to their desks. (Make it Complex)

Simple: Having attended the meeting, they returned to their desks.
Complex: When they had attended the meeting, they returned to their desks.

• Structure:
• Simple Sentence to Complex Sentence: (Having + Verb এর past participle form যুক্ত)
• Structure: Having এর পরিবর্তে When বসে + Subject + had + Verb এর past participle form + বাকী অংশ।

• এ বাক্যে প্রথমেই Having এর পরিবর্তে When বসেছে।
- তারপর Subject - they বসেছে।
- এরপর had + Verb এর past participle form - attended বসেছে।
- বাকী অংশ অপরিবর্তিত রয়েছে।

• এ জাতীয় আরোও একটি উদাহরণ -
- Simple: Having told this, he went away.
- Complex: When he had told he went away.

- Simple: Having done this, he went away.
- Complex: When he had done this he went away.

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