Below are some examples of sentences that I have seen written both ways in professional material. The latter two cases were written by C.S. Lewis in That Hideous Strength (his versions are the sentences without the commas). Lewis seems to omit commas in cases where style guides might recommend them. Which of the two versions for each of these five sentences is correct?
If you send the letter tomorrow, that will be fine.
If you send the letter tomorrow that will be fine.
If they are drinking water, they will be okay.
If they are drinking water they will be okay.
Every month or so over the past few years, Jim sends me an email reminder to check the vendor website.
Every month or so over the past few years Jim sends me an email reminder to check the vendor website.
To a little midland market town like Edgestow, even visitors from the side of the county had hitherto ranked as aliens.
To a little midland market town like Edgestow even visitors from the side of the county had hitherto ranked as aliens. (Lewis)
I know it sounds silly, but in a way I felt sorry for it because it had no hands and couldn't wipe its mouth.
I know it sounds silly but in a way I felt sorry for it because it had no hands and couldn't wipe its mouth. (Lewis)