It actually is different. In English we don't use a case system, so it isn't obvious. But in the Arabic sentence, 'door' is the object. In your English sentence, 'door' is the subject. In English it doesn't matter, but in Arabic it technically does. 'this door is heavy' would be 'هذا الباب ثقيل'. Notice that door has the definite article el-. This connects it to 'this' and tells us that the door is still part of the subject in the sentence. Without the definite article, the subject is merely 'هذا' and we understand the door to be part of the object. I hope this helps.