膝 and 足: knees, legs, and feet
Use surrounding actions to identify the knee and determine whether 足 refers to a leg or a foot.
What rests at the knees
膝, read ひざ, means knee. 膝の上に本を置きました。 means placing a book on one's lap.
膝を床につきました。 says that someone put their knees on the floor. It does not also state that the hands or forehead touched the floor.
A leg or a foot
足, read あし, can refer to a leg or foot depending on context. In 足に靴を履く, the focus is the foot on which a shoe is worn.
椅子に座って足を組みました。 means that someone sat on a chair and crossed their legs. Do not restrict 足 to the ends of the feet in every context.
Part and direction
右足を一歩前に出してください。 asks someone to move their right foot one step forward. Preserve the right side, one step, and forward direction.
膝の下まである靴下 describes socks reaching below the knees. Do not drop 下 and turn them into socks reaching above the knees.
In 右足を一歩前に出してください, which foot moves, and in which direction?
Show the answer and why
The right foot moves one step forward, not the left foot or a step backward.