Let's learn a new case again: the instrumental.

A noun in this case is as if an instrument (hence the name). It is often translated as by, using and with. Here is how the instrumental endings look:

tušinukas : tušinuku (using a pen)
automobilis : automobiliu (by car)
televizorius : televizoriumi (using a TV)
knyga : knyga (using a book)
kėdė : kėde (using a chair)
kambarys : kambariu (using a room)

Notice that a-words (knyga, mama, Milda, etc) stay the same.

Plural:

tušinukai : tušinukais (using pens)
automobiliai : automobiliais (by cars)
televizoriai : televizoriais (using TVs)
knygos : knygomis (using books)
kėdės : kėdėmis (using chairs)
kambariai : kambariais (using rooms)