Question on comma usage

Hi. Would you use commas in the places indicated? As for nos. 1 and 2, I tend not to use commas if the length of the if-clause is not very long or perhaps long since, as I see it, the construction is "I think or believe this" with the word "this" being a noun or noun equivalent (I am not sure I have written what I had in mind correctly, though)

1. I think/believe (the word "that" is optional here, I think) if he had done that (comma?) he would have been hurt.

2. I think/believe (the word "that" is optional here, I think) if he had done what he had been told (comma?) he would have been hurt.

How about the following sentence with a to-infinitive? I have heard that we need not use commas after some very short prepositional phrases (I think I could have heard it wrong, though)

3. To be effective in your study (comma?) you must to this.