door
The word 'Tür' is the German translation for 'door'. It is a common noun and is used to refer to a barrier that is moveable and is used to open and close an entrance or exit of a room, building, or vehicle. 'Tür' is a feminine noun in German, and its definite article is 'die'.
The door is open.
This is a straightforward sentence telling us about the state of a door being open.
Please close the door.
This is an example of a polite request in a formal setting, asking someone to close the door.
I open the door.
This is a simple present tense phrase in German, indicating an action performed by the speaker.