"open up" is used when something is locked or sealed. "the shop will open up at 8 o'clock" "at Christmas the children may open up their presents" Also used to describe removing personal barriers: "as the became friends they opened up to each other". "The lawer got the witness to open up".

we would never say 'to open up' a door. it would mean the door it self would be opened (so you can see inside the door). to open up means to 'reveal the inside of something'... to open means 'to make it be open' (i.e. , in the "open" position ) you do not reveal the inside of the door, so you don't open it up. instead, you are revealing the inside of the room, so you are opening the door so that the room is "opened up". (but we would never say that the room is "opened up", just telling you this so you understand the difference). If the police come go to a criminals house, they might yell "open up!" but they mean to open up *the house* so they can come in. They might also say "open the door!" but they would *never* say "open up the door", unless they want you to cut it in half so you can see its insides :) I think you must become more familiar with it, because sometimes it's hard to find rules which dictate when to use one form over an other, but I think my above explanation fits almost every situation, if not all.

Mauro, They have almost the same meaning. 1) I open the door......I move the door from a closed position. 2) I open up the door.......I move the door (up) to an open position. "Up" here is and adverb that signifies movement "away" from a closed position to an open position. Actually a door opens 'away' not 'up' but the idea of movement toward an open position is still there. -I open up the garage door....If you are talking about a garage door, which opens upward, it makes perfect sense. Just so you don't think that I am making this all up, there is an archaic definition of the word “up” meaning – open. From the OED: up, adverb Of a gate, door, etc.......Open…..now Obsolete. 1390AD Gower, The door is up, and he went in. 1550 Crowley, In service to me no door standeth up… 1480 Henryson, …in he went and left the door up wide. There is a famous movie cliché when the police knock on the door and say,”Open up in the name of the law!"

To add to the others' answers: Open up has a feeling of hurrying the action attached to it. If someone knocks on the door and says "open up." they want you to hurry and open the door.