For an object to be transparent it must have a smooth surface; thus, all transparent objects have sharply focused highlights. This is one of the clues we use to detect transparent objects. A transparent object whose surface is slightly less smooth, like wax paper, frosted glass, or a shower curtain, is translucent. Translucent objects allow light through them but detail is obscured. Their highlights are less sharply focused. Observation and use of these simple phenomena will help you create the illusions of transparency and translucency in your drawings.
Transparent objects have smooth surfaces and focused highlights. Translucent objects, like frosted glass, have unfocused highlights. Details through translucent objects are obscured.