indirect:
He gives the book to María.
DO=Book
Where is the book going?
To María.
IO=María
He gives María the book.
DO=Book
Where is the book going?
To María.
IO=María
Direct:
My mother writes me every week.
DO=letter (understood)
IO=me
(My mother writes me a letter every week.)
She told him.
DO=it (understood)
IO=him
(She told it to him.)