Forms should mostly be self-explanatory.

What we should do:

• always use the right kind of control. Sometimes a checkbox is better, sometimes a field of radiobuttons, sometimes a button or just a link.
• use meaningful and short labels on the controls themselves, instead of long explanations somewhere in the page. This way we can avoid all the conditionals and negation overhead.
• the other way round: if we have an explanation asking to click a link, then make the link part of the explanation.

Checkboxes or Radiobuttons or Selection Dropdowns?

• a checkbox can be used for yes / no decisions. It offers space for an explanation of the "yes" option, but not for the "no" option.
• an array of radiobuttons can be used for choosing between a/b/c/d/e, but also for choosing between "yes" or "no". With radiobuttons, we can write an explanation for every possible option.
• for a yes / no decision where both cases need an explanation, we should use a pair of radiobuttons instead of a single checkbox.
• with a selection dropdown, we can label the options, but there is no space for long explanations. The benefit of selection dropdowns are: less waste of screen space, enough space for tons of options, no distraction by inactive options.
• incremental explanation: the first two or three words in an explanation should be enough to give a rough idea what the option does. A longer explanation can be added in smaller font, in a new line, in a tooltip, or on a new page.

How not to do it

In many places in our site we can still find long and redundant explanations, of the kind

• "if you click the button down there.."
• "if you choose 'no' in 'send me spam about your services', you will not receive newsletters. if you choose 'yes', you will receive all newsletters."

Examples:

• verifymembers page
• personal startpage on test: "Start now to Search for members (by place or username) in the field on the right, add a friend or explore BeWelcome! If you hosting status changed, use the dropdown on the right or go edit your profile"
  • to fix: remove some words, add links

Using such texts is a fatal mistake in a website. They are

• too long, no one reads it.
• in the wrong place, not connected with the control they explain.
• every if/else, and every additional level of negation ("if you NOT choose NO ..") makes the system more complex to understand.

Links and References

•  nielsen, on radiobuttons vs checkboxes long read, can probably be said shorter
•  What Makes a Web Form Usable or Unusable
•  Usability Tips You Can Use: Designing Forms for Keyboard Access is more about labels and shortcut keys

There's a lot more of this somewhere on the web, and in printed literature.