日暮里駅は英語の使い方に注意した方が良い。

I was at Nippori Station the other day, and when I was walking through the station, I noticed this sign on the steps of one staircase.

× No Stair Sky Liner
○ These stairs are not for the Sky Liner.
○ Not the stairs for the Sky Liner.

• staircase【名】〔壁や手すりを含む〕階段 (definition from Eijiro on the Web)

Their phrase puts a noun next to another noun, in this case "stair" and "Sky Liner". When we use one noun in front of another noun, the first noun becomes an adjective to describe the second noun. In their mistake, it sounds like "stair" became an adjective to describe "Sky Liner", which makes no sense.

A correct way to use a noun in front of another noun is like this:

• I always put my computer in a foam case when I'm going to take it somewhere. Some people call this foam case a laptop sleeve.

Notice their other mistake, which is using "stair" instead of "stairs". For example:

× I went up the stair.
○ I went up the stairs.

However, if you use "stairs" as an adjective, drop the "s". That's the rule for using a noun as an adjective: only use singular nouns, even if the meaning is plural.

• A stair climber is a strange kind of exercise machine.

Some days I run up the stairs from the ground floor to the floor my apartment is on. That's good exercise! The next time you are going up a staircase, think about using one noun as an adjective to describe another.