忘れる -合う -換える (compound verbs)
Usage
V忘れる means to forget to do V / V合う means that several people or things do V with each other / V換える means to change something by doing V, or to do V differently
Examples
今日は料理は塩を入れ忘れたので、美味しくない
The food tastes bland today because I forgot to put in any salt
困った時こそ助け合うことが大切だ
When we are in trouble, it is the very time when it is important to help each other
部屋の空気を入れ換えた
I let some fresh air into the room