How will you tell your clients you can't do their tasks without sounding incompetent?

[Btaliat]

Sometimes there are some tasks that may be very difficult to do. Some of these tasks may be out of your range. And in order not to fumble, it is always better to say the truth to the clients. But sometimes, freelancers always have issues saying it. They believe they may sound so incompetent doing that. What do you think a freelancer can do? How do you think he can say it without sounding so incompetent at all?