I'm afraid you're not the only one who's confused, May! Your post is not very clear.
If you have Datasheet selected as the Default View in your form's Property Box the form should open in Datasheet view when either clicking on it in the Objects Dialog Box or when having it open the database thru
Tools -- Startup -- Display Page/Form.
If you open the form thru code in another form, you have to specify that you want it to open in Datasheet View by using acFormDS, regardless of how Default View is set for the form.
DoCmd.OpenForm "YourFormName", acFormDS, , stLinkCriteria
Post back if you have further problems.
Good Luck!