I think you want to say "If you go down tell me so I can open the door for you"

You could say it this way if you want to use because. Tell me if you will go down (stairs?) because I can open the door for you. Tell me if you will go down. (Why tell you?) Because I can open the door for you Eb1 ok you can say it this way too If you go down tell me because I can open the door for you Tell me if you will go down because I can open the door for you. You can also use 'so' like in Kenneth's example or because either way you are giving the reason for why you want to know. You need a verb there like can or will If you go down tell me because I will open the door for you. Hope this helps clarify what is possible.