You may need to set the subform's Default View property to Single Form if you want it to open in the form view.
If you also want to have the option of viewing the form in datasheet view then set its Views Allowed property to Both. If you want only one view, then you can set it to either Datasheet or Form.
If you have a button that opens the form, check the the DoCmd.OpenForm method options. I believe that the option acNormal will open it in whatever you set as the Default View. Otherwise, acFormDS might be declared and could be causing it to open as a datasheet.