The most important clue to provide is a dimmed label for the input field (in addition to, obviously, dimming any default text in the field).
This only works for labels placed (properly) outside the field. A dimmed label or prompt in the field could simply mean “this is not really input.”
You can also try graying the background of the field, changing it to a white background when it becomes editable.
This works best for classic GUI dialog boxes where the window background is the same shade of gray (as in the example above). You want a disabled field to be noticeable but not attract attention relative to enabled controls, like your checkbox. However, a gray field on a white background (as often used in web apps) has strong contrast which tends to attract attention.
You can try “disabled” as a value, but that’s jargon –your users may not know what it means (“No, my email works fine”). You could try a more directive prompt, like “Accept terms of service first.” That's good self-documentation, but adds clutter that, ironically, attracts attention away from where the user should look first (the terms of service and checkbox). If the dimming of the label works for most users, maybe you could present such a prompt only for users that try to put focus on the field.