What's the difference between 开放 (kāi fàng) and 开门 (kāi mén) which both mean "open"?
开门 = open the door

开放 = open to public

开放 means if something is open to public and 开门 means if the DOOR is open, this is the key difference.

For example:

公园开放了吗？Park's open to public yet?

公园开门了吗？Park's open yet?

The first sentence will only be used if the park isn't officially open yet and you won't use it to check park's opening time everyday.

Example when you can use 开放 and not 开门:

经济开放。Open economy.

Obviously you can't use 经济开门 which means Economy open the door