Questions & Answers about Я мою волосы шампунем.
Why is волосы in the plural form? In English we say “hair” singular.
In Russian the word for the hair on your head is normally used only in the plural: волосы. If you talk about a single strand, you use the singular волос, but when referring to all the hair collectively you always say волосы.
Why is шампунем in the instrumental case?
The instrumental case is used to show “with what” an action is performed. Since you wash with shampoo, шампунем (the instrumental form of шампунь) is the correct case.
Could I say Я мою волосы с шампунем instead of using the instrumental without a preposition?
Technically yes, с шампунем (with shampoo) works, but native speakers almost always drop the preposition and simply use the instrumental: Я мою волосы шампунем sounds more natural.
Why isn’t the reflexive verb мыться used here (like Я моюсь шампунем)?
Мыться means “wash oneself,” and usually you say Я моюсь (I’m washing myself). But when you specify the object you’re washing—волосы—you switch to the non-reflexive мыть and treat hair as a direct object: Я мою волосы шампунем.