首 and 肩: neck and shoulder expressions
Distinguish direction from idiomatic meaning in actions involving the neck and shoulders.
Neck movements
首, read くび, means neck. 首をゆっくり回しました。 says that someone turned their neck slowly. ゆっくり specifies the speed.
首を横に振りました。 describes shaking the head from side to side. 横に rules out changing it to an up-and-down nod.
What rests on a shoulder
肩, read かた, means shoulder. 肩にかばんを掛けました。 says that someone put a bag over their shoulder.
右肩に手を置きました。 says that a hand was placed on the right shoulder. Context identifies whose shoulder it is; retain the right-side detail.
Reading idioms
首を長くして待っています。 is an idiom for eagerly awaiting something. It does not describe the neck physically becoming longer.
肩を並べて歩きました。 means walking side by side. 肩を並べる can also mean being on equal terms with a rival, so check the following verb and context.
Can 首を横に振りました be translated as nodding?
Show the answer and why
No. It means shaking the head from side to side. Preserve the direction in 横に.