In the process of <verb> / Have <verb>ed and still <verb>ing (indicates an ongoing state or a prolonged state of the verb).田中さんは 今、勉強しています。
Verb plain form + 前にN4
Before <verb> (indicates an action that occurs before a point in time or another action).寝る前に 歯をみがきます。
Noun + にN4
At <noun> (indicates the place or location where an event occurs).教室に 学生がいます。
Noun + になるとN4
When becoming/when reaching <noun>, then... (indicates the point in time when an event or state occurs upon reaching a certain moment).春になると 花が咲きます。