Learn Spanish (Spain): El cuadro grande está al lado de la puerta.

The phrase al lado on its own means to the side or next door. When you want to say next to a specific object, you must add de (of). So, next to the door translates literally as at the side of the door: al lado de la puerta.

Does the al in al lado de change since la puerta is feminine?

No, the phrase al lado de is fixed. The al is a combination of a and el (to the) which matches the masculine noun lado (side). Because lado is always masculine, the phrase stays exactly the same no matter what object comes next.

Why do we use está instead of es for the painting?

We use está from the verb estar because the sentence talks about a physical location. In Spanish, you always use estar to state where something is located in space, even if it is a heavy object that rarely moves.

Why is it cuadro grande and not grande cuadro?

In Spanish, descriptive adjectives like grande (big) almost always come right after the noun they describe. So instead of big painting, the sentence is structured as painting big (cuadro grande).