It is many years since I _____ to London.

• Since এর প্রথম অংশ Present Indefinite/ Present Perfect Tense হলে পরের অংশ Past Indefinite Tense হয়। 
- আবার Since এর প্রথম অংশ Past Indefinite Tense হলে পরের অংশ Past Perfect Tense হয়। 
যেমন: 
- Many years have passed since his father died. 
- It was many years since they had first met. 

• প্রদত্ত বাক্যে since এর পূর্বের অংশ It is many years অর্থাৎ Present Indefinite Tense বিধায় পরের অংশ Past Indefinite Tense হবে। 

Complete Sentence: It is many years since I came to London. 

Source: Advanced Learner's English Grammar & Composition by Chowdhury & Hossain.