When the subform, or form, is in Continuous or Single view it seems to
understand where I am in the form; when I'm on a form in datasheet view it
gets lost.
I'll have to do some experimenting, but IME you can refer to
Me![fieldname] - no aliases, using the name of the field rather than a
separate name of a control - to update a selected field in a table.
But I don't use datasheets very much so I may be mistaken here!
One suggestion: *why* a datasheet? You can make a Continuous Form look
exactly like a datasheet if you like the appearance; about the only
thing you lose is the ability of the user to resize columns.