(Enter, go in) To go into or enter a space. Though commonly read はいる in modern speech, this reading (いる) appears in set phrases and literary contexts.
客が部屋に入ると、皆が挨拶した。
When the guest entered the room, everyone greeted them.
祭りの行列が町中に入ると、人々は拍手した。
When the festival procession moved into the town, people applauded.
その言葉は彼の胸に深く入った。
Those words deeply entered his heart (had a deep impact on him).
(Of the sun/moon) to set or sink below the horizon; to go down (used often in literary/poetic contexts).
夕日が西の山に入って、空が赤く染まった。
The sun set behind the western mountains, dyeing the sky red.
満月が雲に入って見えなくなった。
The full moon sank into the clouds and became invisible.
To attain or reach a state (e.g., nirvana, enlightenment, a climax); to enter into a particular state or condition.
長年の修行の末、彼はついに悟りに入ったと言われる。
After years of training, he is said to have finally attained enlightenment.
物語はクライマックスに入るところだ。
The story is entering its climax.
A suffixal use after the -masu stem of verbs meaning to do something fully, intently, or sincerely (e.g., 聞き入る listen intently; 感じ入る be deeply moved).
みんなは彼の話に聞き入った。
Everyone listened intently to his story.
その演説には多くの人が感じ入った。
Many people were deeply moved by that speech.
A suffixal use after the -masu stem meaning to reach a state completely or to do something thoroughly (e.g., 寝入る to fall completely asleep).
赤ん坊はミルクを飲むとすぐに寝入った。
The baby fell asleep immediately after drinking the milk.
長い会議の後で、彼は椅子でうとうとと寝入ってしまった。
After the long meeting, he dozed off and fell asleep in his chair.