Open the door
Use this formal version when speaking to someone you don't know, an older person, or someone in a position of authority (e.g., 'usted').
A polite and common way to ask someone you know well to open the door. It means 'Can you open the door, please?'.
Use this when giving a command to a group of two or more people ('ustedes').
It's hot in here. Open the door, please.
Excuse me, sir, can you open the door? My hands are full.
Kids, open the door for your grandmother!
Honey, I'm getting to the car. Can you open the door for me?
The phrases 'Abre la puerta' (informal 'tú') and 'Abra la puerta' (formal 'usted') are the standard and are understood universally.
In countries that use 'voseo', the informal command is 'Abrí la puerta'. The stress moves to the last syllable: 'ah-BREE'.
To address a group of people informally, speakers in Spain use 'vosotros', making the command 'Abrid la puerta'.
Saying 'Abrir la puerta' as a command. 'Abrir' is the infinitive ('to open'). The correct command form is 'Abre' (informal) or 'Abra' (formal).
Using the wrong gender for 'puerta'. 'Puerta' is a feminine noun, so you must use the article 'la' ('la puerta'), not 'el puerta'.
Using the informal 'Abre' in a formal situation. It can sound too direct or even disrespectful. When in doubt, use the more polite formal command 'Abra la puerta'.