What clauses and long phrases

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- What a friend of mine did was interesting. 

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What is the subject of WAS in the sentence? 

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ANSWER: What a friend of mine did. This is called a WHAT CLAUSE. 

​- The first thing to do is to decide on an area you are interested in. 

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What's the subject in this sentence?

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ANSWER: The first thing to do. This is called a NOUN PHRASE.

WHAT CLAUSES and LONG NOUN PHRASES

introduce important information.

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• ​What she has are some good memories.

• ​What they found in that company is amazing.

• ​What I will do next year is (to) start my own business.

• ​What my father doesn't know is (that) I got a new job.

• ​​The best thing she has are some good memories.

• ​The information they found in that company is amazing.

• ​The new thing I will do next year is to start my own business.

• ​Something that my father doesn't know is (that) I got a new job.

Check these sentences:

- What the plan does is to ensure a fair pension.

- What the plan does is ensure a fair pension.

Both are grammatical correct, and there's no difference in meaning. The version with "to" is somewhat more common in both American and British English.