The way you make such a triangle is to start with an equilateral
triangle with three 60° angles



Then we cut it in half, making two 30° angles
at the top:



Look at only the right triangle on the left:



The red side is half of the green side because
all the sides were originally the same since it
was an equilateral triangle.  We cut it in
half, so the red side is one-half of the green 
side.  That makes the green side twice the
red side.



The red side is the short side and the green side
is the hypotenuse.  So your sentence should read:

In a right triangle with one acute angle that is
30 degrees, the hypotenuse is        twice        
the short side.

Edwin