Is it acceptable to begin a declarative sentence with “Am”?
Is it correct to start a sentence with did.
Did my work.
I did my work.
while answering my emails?
You can do this if being informal, especially in spoken English. Example:
But to be formally correct, you need to include the personal pronoun:
When answering an e-mail, I'd recommend sticking to the formal version, unless you're explicitly speaking douchebag.
Even if you get an informal e-mail from your superior, I would still recommend being at least grammatical in your reply. You don't necessarily have to be formal as if addressing the king, but you should still at least show your intelligence level by not breaking the basic grammar rules.
The version of Eng. is pertinent. In Brit. Eng., the the question What have you done today? would be answered with I've done/gone/seen/.. In Am. Eng., the question is more likely to be what did you do today? the answer would be I did/went/saw...
In Am. Eng. and in many languages of the world the past simple tense is used to indicate a recent event that has an influence on the present. In Brit. Eng, we prefer the present perfect tense - after all, that is the purpose of the pres perf.
American/Nepali/Indian child weeping: I fell over! Brit child weeping: I've fallen over!
When abbreviating language, you can do anything you like if think the reader will understand. So, choose 'Did' or 'Done' as appropriate.