Akash has been practicing the guitar since he (receive) it as a birthday gift.

1) Any Present tense + since (সময় বুঝাতে) + Past Indefinite
Example:
- It is ten years since his father died.
- Many years have passed since his father died.

2) Past tense + since + Past Perfect
Example:
- It was ten years since his father had died.

• যেহেতু প্রশ্নে উল্লেখিত বাক্যে since এর পূর্বের clause টি present perfect tense এ আছে, তাই এর পরের cluse এ verb টি past indefinite এ হবে।
- Complete sentence: Akash has been practicing the guitar since he received it as a birthday gift.