Since a face doesn't exist independent of it's body, I'm not surprised that you can't save it as a separate file. You could insert the Part containing the face into a new Part, and reference it that way, if that will work for your situation.
Unless you mean that you have multiple surface bodies. Each one being its own separate entity. Then you should be able to select the body in your tree and select "Insert into New Part..."
If what you really want is to save a single face, here's what you can do:
1) Turn on the "Face" Selection Filter:
-It should be on the selection filter toolbar:
- Note: if you can't find this toolbar, go to View -> Toolbars -> Selection Filter and enable it.
2) Select All: Edit -> Select All; or, CTRL + A
3) Start the "Delete Face" command: Insert -> Face -> Delete
4) Deselect the face you want to keep.
- BEFORE completing the Delete face command, deselect the face you want to keep by left clicking it once.
5) Complete the "Delete Face". Click the green check mark. You should be left with a surface that is just the face that you wanted to keep.
i begins learning solidworks and try to make a bmw z4 3d surface car
i mean that is surface