Using a comma between the subject and the verb

Question

Except when used with parentheticals, I was taught that I should not put a comma between the subject and the main verb used in a sentence.

Io, ho fatto in modo che tu fossi assunto.

Is that always not correct, or should I use the comma in some contexts, for example to put the emphasis on the fact it was me to do something?
My reasoning is that, since ho fatto in modo che tu fossi assunto is already a sentence, io is just an addendum I could use to put emphasis on the person who did something, similarly to what English does with "I did help you with getting the job" used instead of "I helped you with getting the job."

Answer 1

Putting a comma between the subject and the verb for emphasis or to render the pauses in speech is a common mistake, but it's still wrong; depending on the context, you could render the emphasis in different ways:

On an Internet forum, in IRC and the like I would probably emphasize the "io" with formatting, or with all caps:

Io ho fatto in modo che tu fossi assunto

IO ho fatto in modo che tu fossi assunto

In a book, the effect may be rendered by breaking the quotation appropriately; italic is also appropriate:

«Io» sottolineò «ho fatto in modo che tu fossi assunto»

«Io ho fatto in modo che tu fossi assunto»

Or you could just rephrase the sentence to make it both more emphatic and in a way easier to render in written form:

Sono stato io a fare in modo che tu fossi assunto, non dimenticarlo.

Sono stato io - io, non dimenticarlo - a fare in modo che tu fossi assunto.

(notice that both these forms are quite rude and arrogant, the natural evolution of the story in my opinion would be

)

Answer 2

Io, ho fatto in modo che tu fossi assunto.

is not orthographically correct.

Maybe, in the right context, you could say:

Ho fatto in modo che tu fossi assunto; io!

thus emphasizing that you, and not another person or the listener themselves, had an important role in their career.