Observation
Shash tshinipati tekushinian. You were already asleep when I arrived.

The Independent Indicative Past conjugation (#03), like the form tshinipati above,  is used for things that happened in the past.

Shash nipapan ka takushinian. S/he was already sleep, when I arrived.

It can be used with the modal preverb tshipa.

Shashish an tshipa tshitutepan uipat aieshkupitakue. S/he would have left if I had arrived.

It can be used with the volitional preverb ui.

Shash tshui nipati …tekushinian. You already wanted to sleep / You were about to sleep …when I arrived.
Shash ui nipapan …tekushinian. He already wanted to sleep / He was about to sleep …when I arrived.
Shash an tshipa ui tshitutepanat utakushit. They should have already left yesterday.

More on preverbs

Link to the conjugation guide.