Please stretch before getting in the pool.
Please enter from the entrance.
Let’s go in the room and draw our eyebrows on!
When a kanji ends with an "う" sound (like る, for this one), it's usually going to be a verb. When the vocab is a kanji + okurigana ending in this "う" sound, it's often going to be the verb form of that kanji. So in this case, the kanji 入 is enter. The vocab 入る is to enter. Just the verb version of the kanji.
When a vocab is a single kanji with okurigana, chances are it's going to be the kun'yomi reading, which you don't usually learn in the kanji (you usually learn the on'yomi reading). So, let's learn the kun'yomi reading for this word using its meaning.
When you enter the room, what do you do? You say "Hi" (はい) to everyone to the point of absurdity. "Hi there. Oh hi Frank. Oh hi there Susan. Oh hi there lamp."