Learn Spanish (Latin America): Ella está delante de la puerta.

Both mean "in front of", but they have a slight difference. delante de usually means "ahead of" or "at the front part of" something, like standing right at a door. enfrente de usually means "facing" or "opposite" something, like a building across the street.

Why do we use está instead of es in this sentence?

We use está (from the verb estar) because we are talking about a physical location. In Spanish, you always use estar to say where a person or thing is located right now.

Would de la change if she was in front of a masculine noun, like a building (el edificio)?

Yes! If the noun uses el (the), the words de and el combine into the contraction del. So "in front of the building" becomes delante del edificio. Because a door is feminine (la puerta), the words stay separate as de la.

Do I have to include Ella at the beginning?

Not if the person you are speaking to already knows who you are talking about. Spanish verbs change based on who is doing the action, so if the context is clear, you can simply say Está delante de la puerta.