English is annoying and often doesn't make much sense yes 🤣. It's OK, don't get frustrated. Sometimes it's more about how you are saying it that matters depending on the context. To speak with style and eloquence is something most native English speakers struggle with so don't worry 🙂 Everything is OK and you will be fine. Keep going and keep learning. Hope this helps. Adios :)

Great question! 'Key' has two different meanings, and the form of the verb, infinitive (to open) or gerund (opening) helps us understand which meaning is intended. Key can be a physical object (una llave) or a concept (una clave) (something important that will help me to complete/achieve something). 'Key to + openable object/open' tells us we are talking about the physical object (llave), and 'key to + gerund/conceptual noun' tells us we are talking about the concept (clave). Let's use your examples in context. 'The key to open the door' means 'la llave para/de la puerta', the physical object that I put in the lock to open the door. Here 'to' means 'con el fin de, para (lograr hacer algo)'. Ex. The key to open the door is on the table. I can't find the key to open the door. 'The key to opening the door' means 'la clave para abrir la puerta'. Maybe the door is difficult to open, and there is a specific motion you need to do to make it open. Ex. The key to opening the door is to turn the doorknob as you turn the key, otherwise it won't open. The key to opening the door from the outside is to pull the handle towards you until you hear it click. In English, we often use the gerund form and the infinitive form of the same verb to express different ideas. Natives learn this naturally from context, but you can definitely learn it with time and lots of examples.:)

I lost the key to open my door The key when opening a bank account is to ask ….. I know English is not so simple sometimes but it’s way easier than Spanish 😉