Long Riddles and Answers

Question: No matter how hard you try,
I can open what you can't.
If it wasn't for me,
on your front porch you would stand.

What am I?

Question: Something different to show every week, but I'm still the same place.
If you're eating it's probably butter that you taste.

What am I?

Question: I am unseen but all people know of me. I am intangible yet sharper than any sword. I come from nothingness but can fell even the mightiest kings.

Question: Those who make this item, don't use it. Those who BUY this item, have no need for it. But those who USE this item, never see it. What is the item?

Question: Sophie who is a housemaid, knows her boundaries well. She is good at equations. Her favorite vegetable is cauliflower. Which is her favorite plant?