Using IWPParInterface you get access to the current paragraph. You can change the text or change the attributes.
IWPParInterface is also used to modify paragraph styles. Here you cannot add text but You can change the attributes.
This three basic function IWPAttrInterface.AttrDel, IWPAttrInterface.AttrGet, IWPAttrInterface.AttrSet will modify the character attributes for IDs <=16, for IDs >16 the current paragraph / selected paragraphs will be modified. You can use Memo.TextAttr.AttrSET((int)WPAT.IndentLeft, 360) to change the indent of the current paragraph or all selected paragraphs, in case text was selected.
IWPDataBlock provides low level access to the paragraph tree. You can use it to append paragraphs and to delete header or footer.
IWPTextObj provides access to objects such as field markers and hyperlinks, but also images.
IWPNumberStyle can be used to create and modify numbering styles.