While I (study) at the library, I found a forgotten notebook.

• While:
Bangla Meaning:  যে সময়টায়; ততক্ষণ-যতক্ষণ; ঠিক এই যা সেই সময়ে, একই সময়ে।

→ While (as a noun) a period of time especially when short and marked by the occurrence of an action or a condition.
→ While (as a conjunction) during the time that; as long as.
→ While (as a preposition) until.
→ While (as a verb) to cause to pass especially without boredom or in a pleasant manner.

• While দ্বারা দুইটি বাক্যাংশ যুক্ত হলে, While এর পূর্বে অংশটি Past Indefinite Tense- এ হলে, এরপরের অংশে Past Continuous Tense হয়।
• While- এর পর অনেক সময় Auxiliary Verb না বসে সরাসরি Continuous form বসে।
• অতএব, নিয়মানুযায়ী সঠিক উত্তর হবে - was studying.

Complete Sentence: While I was studying at the library, I found a forgotten notebook.

Example Sentence:
1. My uncle arrived while I was cooking the dinner.
2. While I was reading, the phone rang.
3. She sang a song while washing the dishes.
4. The children laughed while playing in the park.