By Infinitive clauses, Commas

Do you use a comma if it's in the middle of the sentence?

To err is human. (No comma) To respect the elderly is our duty. (No comma) To tell the truth, I’m angry. (Comma req’d?) To see is to believe. (No comma) To make a mistake carelessly is inexcusable. (No comma) To make a long story short, I don’t know the ans. (Comma req’d?)

Or is there certain amount of words it has to be like for prep phrases?

1-- Yes, you can.2-- Interestingly, I thought a good while before using that comma, and I like your way just as well. In the end, I used the comma to make the infinitive clause more appositive to the main clause; but frankly, I am not sure how I should connect it to the sentence. For me, it...