のは

The Who, What, When, Where, and Why
Plain form verb + のは

の in のは is a stand-in for some other, known, obvious word. It’s sort of like “it” in English, but it must follow a verb and it will usually be in the のは pattern. 

のは typically represents the Who, What, When, Where, or Why of a sentence.
のは frames a sentence as answering an implied question.
“The one who did it was him.” “The reason why is because..” “The thing I did was..”

Be careful as のは may be the nominalizing の + は particularly when followed by an adjective.

Why (Reason)

This meaning in particular tends to be used to answer a question. 

• 僕がやったのは、君を助けるためだった
  The reason I did it was to save you.
  
  
• 牛を飼うのは、牛乳と肉を得るためだ
  The reason we keep cows is for milk and meat.
  
  

What (Thing)

• 食べているのはウナギだ
  The thing that I’m eating is eel.
  
  
• 一番好きなのは、やっぱり猫だ
  The thing that I like the most is (after all) cats.
  
  

Where (Place)

• 彼が行ったのは学校ではなく、ゲームセンターだった
  The place he went was the arcade and not school.
  
  
• 魚がたくさんあるのは、湖の北のほう
  The place with a lot of fish is the north of the lake.
  
  

Who (Person)

• 彼が殴ったのは犯人ではなく、隣の人だった
  The person he hit wasn’t the culprit but the person beside him. 
  
  
• 謝るのは僕じゃなくて彼だ
  The one who should apologize isn’t me, but him.
  
  

When (Time)

• 授業が始まるのは5分後だよ。
  The time when class is going to start is in 5 minutes.
  
  
• ご飯を食べたのは24時だった
  The time I ate dinner was midnight.
  
  

With Questions

With question words, there’s more emphasis on the question (before のは)  than the “who/what/when etc.) 
This is because you make the statement first, then ask who, instead of ask who then make the statement. This isn’t a large difference though. 

• 誰が僕のパンを食べた？
  Who ate my bread? (Comparison sentence)
  
  
• 僕のパンを食べたのは誰だ？
  Who ate my bread? (Someone ate my bread. Who did it?)
  
  
• 勉強したのはいつだ？
  When did you study? (You studied when?)
  
  
• 行くのはだれだ？
  Who’s going to go?