Let's remember a verb that we have learned in a previous lesson:

duoti (to give)

Usually, when we use a verb like give, we speak about direct and indirect objects:

I give a pencil. You give a book. He gives a pencil to his brother.

In Lithuanian, these objects change their endings:

Aš duodu pieštuką. I give a pencil.
Tu duodi knygą. You give a book.

The new ending that you see is the accusative case. The nominative case of pencil and book is pieštukas and knyga, respectively.