Some hard-and-fast rules on infinitive usage

In 2004 a Forum member based in New Jersey raised a very insightful grammar question on infinitive usage that I think would be a very useful guide to all those aiming for greater mastery of their English.

Douglas Maliszewski wrote me by e-mail: "As a wordsmith junior class, I would rarely question you. However, in your column 'The purposive ways of viewing the future – III,' I wonder if the sentence 'What everyone will be doing in the next five months is size up the candidates' will be better said using the to-infinitive: 'What everyone will be doing in the next five months is to size up the candidates.' It feels more natural, sounds more correct to my ear."