Unstressed words are often left out at the beginning of a sentence if the meaning is clear without them. Words that can be left out include auxiliary verbs, personal pronouns, possessives and articles.
Kids are studying. (= The kids are studying.)
Seen John? (= Have you seen John?)
Wife is unwell. (= My wife is unwell.)
Couldn’t do anything. (= I couldn’t do anything.)
Must go. (= I must go.)
Can’t wait for the holidays. (= I can’t wait for the holidays.)
Will do it tomorrow. (= I will do it tomorrow.)
Doing well, I hope? (= You are doing well, I hope.)
Nobody at home. (= There is nobody at home.)
Careful what you say. (= Be careful what you say.)
Be forty dollars. (= That will be forty dollars.)
See you soon. (= I will see you soon.)
Haven’t seen him. (= I haven’t seen him.)
Auxiliary verbs can be left out before personal pronouns except I and it.
He want to come? (=Does she want to come?)
You ready? (= Are you ready?)
But you can’t say: I late instead of am I late?