Meine arme or mir die arme?
I understand that in German, Im washing my hands would go like "Ich washe mir die hande" but I checked google translate for other body parts and the body itself.. and apart for "mir den kopf" I get "meinen arm/mund/korper, meine zahne/fuße, etc." Isnt the dative reflexive used with these as well?
You can probably use dative for these too. Google Translate is not so reliable, as it translates a little close to word for word so languages that rely on cases a lot don't translate as correctly with Google Translate. I reckon you could say 'meinen...' as it uses the accusative, but yeah the others should follow dative too. Correct me if I'm wrong, though.
I'm betting there is a general rule. "sich(Dat.) etwas waschen" = "To wash one's something"
You can also say "I got a haircut" with "ich habe mir die Haare schneiden lassen." So "sichDat. die Haare schneiden lassen" = "to get a haircut"
Does this only work for bodyparts? How many other verbs work with "sich(Dat.) etwas" in this way? I'm wondering if there is a general rule that can be applied here to other verbs.
Also clothing: sich etwas in die Tasche stecken, for example, "to put something into one's pocket"; eine Taube hat mir auf den Hut gemacht "a pigeon went potty onto my hat".
The line to "dative as benefactive" is blurry -- you also have things such as sie hat mir ein Buch gekauft (she bought a book for me) or ich stelle es ihm ins Auto (I'll put it into his car [for him]).
Nah, works just for your body parts. Otherwise it would sound weard. "Ich wasche mein Auto." "I wash my car" "Ich wasche mir mein Auto." would mean something like "I'm washing me my car." BUT AGAIN no one will ever say that and it's just wrong! It's bad English and it's bad German, too.
Both is correct! "Ich wasche mir die Haare." and "Ich wasche meine Haare." Works for other people to "Ich wasche dir die Haare." and "Ich wasche deine Haare." It can be said that the focus of the first sentance "Ich wasche mir die Haare." is a wee bit more on what I'm doing (the washing) and the focus of the second one ("Ich wasche meine Haare.") lays more on the fact that it is my hair, that I'm whasing. But again both is correct and whatever you choose no one from Germany will ever (!!!) complain that it is a badly build sentance whatsoever!!! No one will even recognise the difference if you hide both sentences in a communication!!! (Sorry I cannot explain it gramaticaly; and I really don't know if there is even a rule for that; almost sure there isn't)