As a non-native English speaker I face difficulty in understanding about shall vs will in interrogative sentences. can you please suggest which one is correct?
1. will i do?
2. shall i do?
Why we use only shall not will in the above sentence?
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Will I do it?
This is, as you said, never used. Modal verb "will" represents a way of asking someone else to do something (for you) or offering them something or your willingness to do something for them.
will (REQUEST) - used to ask someone to do something; used as a polite way of inviting someone to do something, or of offering someone something Source
We do not use it in an interrogative sentence using the first person pronoun "I".
Shall I do it?
"Shall" is used used when referring to the future instead of "will," especially in questions/interrogative sentences including first person pronoun "I" or "we". Also "shall" has its fair share of uses when you want to make more formal comments and statements. It is also a form used to make requests, as your sentence indicates.
Note: In the past, as taught in schools, the future tense in English was formed with "shall" in the first person – I shall go, we shall go – and "will" in the second and third persons – you will go, Mary will go, they will go. In modern American English, "will" is commonly used in speech and writing for all three persons – I will go, etc. "Shall" is used mainly in formal situations with the first person – We shall be pleased to accept your invitation – and in legal documents. Source
The above sentences make sense when writing affirmative sentences. However, those traditions are not followed when you are constructing interrogatives.
[Minor edits: We capitalize "I" no matter where it appears in the sentence. Also, the first letter of the first word of the sentence is capitalized.]