Breakdown of Врач сказал мне лечь на правый бок.
я
I
врач
doctor
на
on, at / by (transport) / to (a place) / to (in answer to)
сказать
to tell, to say
Questions & Answers about Врач сказал мне лечь на правый бок.
Why is it на правый бок in the Accusative case instead of the Prepositional case?
Even though you end up resting on your side, the verb лечь (to lie down) describes a motion or a change of position. In Russian, when you move onto or into a position, you use в or на with the Accusative case. If the sentence described where you were already resting, you would use the Prepositional case: на правом боку.
Why use лечь here instead of лежать?
Лежать describes the state of being flat (to be lying somewhere), while лечь is the physical action of getting into that position (to lie down). Since the doctor is telling the patient to actively move into a new position, we need the action verb лечь.
Why is the perfective verb лечь used instead of its imperfective pair ложиться?
The doctor is giving a specific instruction for a single, one-time action that focuses on the end result (getting onto the right side). When referring to a single, completed action like this, we use the perfective aspect.
Why does the sentence say сказал мне instead of using the Accusative меня?
The verb сказать (to tell or say) requires the Dative case for the person receiving the information. You are saying something to someone, so the pronoun я (I) changes to its Dative form, мне.