when to use "lavarse" versus "lavar"?

Question

don't both mean "to wash"- what is the difference?

Answer 1

It is really quite simple.

Figure out what the subject is. Then figure out what the direct object is.

If the subject is the same as the direct object (even if only partially) then you use lavarse.

So, I wash my car, lavo mi coche.

I wash my arms, me lavo los brazos.

As a side note, most parts of the body, in fact probably all of them, are referred to with the article (el/la) rather than a possessive adjective (mi/tu/su) in Spanish. This is why you say "los brazos" instead of "mis brazos." Kind of like in English when a doctor talks about your inner body parts, sometimes he'll say "the femur has cracked" or something like this.

Answer 2

You want to wash yourself or your clothes?

Te lavas las manos (lavarse) you wash your hands
Lavas la ropa (lavar) you wash the clothes