Verb + 着 (ongoing state)

Frozen in an ongoing state

• (ongoing -ing)着zhe freezes an action into a state that lasts.
• 门开mén kāi(ongoing -ing)着zhe = "the door is (standing) open"; station站zhàn(ongoing -ing)着zhe = "standing".
• It's the "-ing" of a held position, not a passing action.

Verb + (ongoing -ing)着zhe

• Verb + (ongoing -ing)着zhe = the action's resulting state continues.

door门ménopen开kāi(state)着zhe。

• 门开mén kāi(ongoing -ing)着zhe = "the door is open" (and stays open).

Two things at once

• V1 (ongoing -ing)着zhe + V2 = do V2 while in the V1 state.

he他tāsit坐zuò(state)着zheread看书kàn shū。

• to sit坐zuò(ongoing -ing)着zheto read看书kàn shū = "reading while sitting."

Recap

Read the full reference: HSK 3 grammar — Particles.