I forgot to return my i94 yesterday and I am planning to revisit USA in December . What will happen when I try to re-entry

I'll go slightly further and suggest that not returning the I-94 is in fact normal, and is exactly what frequent users of the land border would be expected to do. If you were to cross the land border again with the still-valid I-94 your new entry would be treated as a continuation of your previous stay and you'd be waved through without having to go inside to fill out and pay for a new piece of paper. For, say, a once-a-month (or week) land border crosser keeping the I-94 is a significant benefit both to the traveller and to the CBP since they needn't bother processing new paper on each and every entry. While the policy of treating subsequent reentries from Canada/Mexico as continuations of a previously granted stay is usually argued here to be a defense against visa runs I suspect the more important reason for the policy is to optimize land border traffic (where 80% of the entries by foreigners may occur).

You are supposed to return the I-94 before travel outside of North America, or before it expires. If you live near an airport with US preclearance the CBSA office there may accept the form. I don't know what happens when you fly to the US in possession of the form, which is kind of out of place there, but I imagine the CBP officer would just pull it himself if he's giving you a new stay.