Re: "No problem"
I have to go on record to state my objection to "no problem" as a response to "thank you." Yes it's common but I find it rude and graceless.
When a server in a restaurant gives me my meal and I say "thank you" and I get "no problem" as the response I have to fight the urge to say "I know it's not a problem. In fact it's what you're getting paid to do."
A local fast food place has trained their employees to always respond with "It's my pleasure." I appreciate that.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.