Which is grammatically correct - "define" or "to define"?

Question

I am writing a description for a video. I want to know which of the following is more correct:

1. The next step is to define the cross section of the member.

2. The next step is define the cross section of the member

Answer 1

Number 1 is correct because you want to use it as an infinitive verb. Here is a brief explanation with some examples:

There are several possible ways to use infinitive verbs. You can use them:

1. as the subject of a sentence – To err is human; to forgive, divine.
2. like an adjective or adverb phrase that expresses purpose or intent – My instructions are to press this button every hour.
3. following an indirect object – He told me to give this to you.
4. following certain other verbs - source

Answer 2

Number 1 is correct. The only way you can make 2 correct is if you were reading a list of instructions such as:

1. Get milk.
2. Get eggs.

And then the person reads the list and says: "Step one is 'get milk', step two is 'get eggs'".

Answer 3

#1 is correct. #2 could be correct if you change "define" to "defining". "Defining" is a gerund noun, and a noun works fine here.