Hi I am new to this website and hoping someone show me the corerct usage of "I" and "me".
Do I write "Let Jack and I know when you will be here" or "Let Jack and me know when you will be here".
You should use 'me' in the sentence you provided.
A simple check can be carried out to decide whether you need a subject pronoun such as I, he, she, etc. or not: Try to use 'we' instead of jack and I/me if it doesn't sound right try using 'us'. If 'us' works, then you need to use 'me'.
Take Jack out of the sentence and see what you get!
Would you say: "Let ME know when you will be here." Or, would you say: "Let I know when you will be here."
The correct pronoun to use is me.