Please, all of you beginners out there, do NOT straighten your arm, I beg you
Horrible advice.
All players should hit with double bend forehands and they do. In case of Roger and Rafa, the arm is "straighter" but it is bend. NEVER lock your elbow when hitting the ball, not only you will have almost 0 control, you can also injure yourself.
They images chosen for Rafa and Fed above give the illusion that the arm completely straight and elbow is locked. This is not the case. It certainly is straighter then most, but there is a slight bend in the elbow, this allows them to engage the bicep. Try to lock your elbow all the way out, then contract your bicep - what happens? ....a slight bend.
But that is not even the point. You should learn a proper forehand and your coach should shape it around what is natural to you. Attempting to replicate something that is unnatural to your body will not result in better performance, quite the opposite.
In fact, I think many of you likely to have a large number of quirks on your forehand that when fixed will give you much better results then trying to straighten the arm on Forehand.
Place your off arm on the throat of the racquet when making a preparation to hit a forehand, keep it there as you reach your hitting position and find the ball. Let go and straighten the non hitting arm parallel to the baseline as if you were trying to catch the ball with your pinky up. Take your racquet back with racquet-head pointing up and your wrist extended. Make sure you grip the racquet VERY lose. Initiate your shoulder turn into the ball by bending the non hitting arm slightly at the elbow while opening up your shoulders. Drop the racquet-head below the path of the ball while keeping your wrist slightly extended. Rotate your body into the ball while keeping your arms in a hug position. Slight lag on the hitting arm and keep your wrist extended, point the butt of the racquet towards the ball. Continue accelerating the racquet at and after the contact. Release the extension of your wrist at contact. Let the racquet arm wrap around you, don't attempt to slow down or stop it.
This is a better angle to show the arm on Federer:
Level 13 Edberg and counting...