Apri il portone con la chiave.

Apri is the second person singular imperative (tu) of the verb aprire (to open). In Italian:

• For -ire and -ere verbs, the tu imperative is identical to the present indicative: tu apri → Apri!
• For -are verbs, the tu imperative drops the final -i of the present indicative (e.g., parli → Parla!).

Use this form when you tell one person (informal) to do something.

To address someone with Lei (formal you), switch to the third person subjunctive used as a courtesy imperative:

• Apra il portone con la chiave.
  You can also soften any command by adding per favore (“please”):
• Apra il portone con la chiave, per favore.

For a negative tu command, use non + infinitive, not the imperative form:

• Non aprire il portone con la chiave.
  Note: You place non before the infinitive aprire, and you don’t attach pronouns in the same way as with affirmative imperatives.

The direct object pronoun for il portone is lo.

• Affirmative tu imperative: attach the pronoun to the verb → Apri​lo con la chiave.
• Negative tu imperative: place the pronoun before the infinitive → Non lo aprire con la chiave.

• porta: a standard door (e.g., room, house, car)
• portone: a large or main entrance door (often of an apartment building)
• cancello: a gate (typically metal or wooden, part of a fence or entryway)

Choose the word based on size and context.

Con indicates the instrument or means: “with the key.” Alternatives include:

• usando la chiave (using the key)
• per mezzo della chiave (by means of the key – more formal)
  But con is the most direct and common way to express “using” or “with.”