Are 'tvättar' and 'vatten' related, etymologically speaking? 'Tvättar' seems kind of like 'wetting'...
Exactly the way it is spelled. ä is fairly close to "a" in "apple" (in some english dialects)
It depends on dialect, although yours is a better general approximation.
My first guess for 'tvätter' was "cleaning" instead of "washing". What's the difference in nuance and what would be a correct translation for "cleaning"?
Cleaning would rather translate to städa, as in tidying up and the like. However, there is definitely some cleaning in tvätta too, but usually it has to do with water somewhere.
Sina/sitt Is a construct I have only seen in the Nordic languages. It is a reflexive pronoun that reflects back the third person (sing or plur) , just as Zmrzlina points out below. What does this mean then? I'll give you an example:
Han tvättar sina kläder - His is washing his clothes (his own ones) Han tvättar hans kläder - He is washing his clothes (someone else's)
We don't have this in 1st or 2nd person.
They have something very similar in Slavic languages. In Russian, it's свой, svoj. (then declined in all genders and cases). In Russian though, свой can refer back to the first person too. It's optional to use it for 1st person, but necessary in the 3rd.
Baltic languages as well, and it's not really optional in sentences like this. (As far as I know, it's actually necessary in Russian 1st and 2nd person also, but I could be wrong.)
I don't understand this "optional/necessary" business. свой literally means "one's own", which is not the same as just "one's" of the regular possessives
Thank you! I feel all of my explanations in this thread are a bit rambling …
You're correct, it is obligatory to use the reflexive possessives for the third person in Swedish. If you say Han hittade sin bok, he found his own book, but as soon as you say Han hittade hans bok, the book he found cannot be his own, it has to belong to some other male.
This system is a bit weak in some places and you can definitely hear native speakers make mistakes, but it's still safe to talk about right and wrong here, although it may well be that the system will look different in fifty years time.
At least my teachers used to tell me that it isn't wrong to say я взял мое [whatever], it's just that it puts a certain special stress on the whatever being 'mine'. Which means that using свой in that case is not obligatory. While with her or him it's obligatory or meaning changes into the thing belonging to someone else. Например можно сказать Я нашел мою книгу, а не твою.
For some reason I can't reply to Arnauti's post below (no Reply link), so replying here: You're correct, and your explanation is very good (although I would put your example after the first sentence since it illustrates what you said in the beginning). Now back to Swedish: is it obligatory to use reflexive possessives in Swedish?
sure, my point is that sin cannot point back to the first person in Swedish. Nor to the second. Just the third.
It is quite probably related to the other "reflexive particle" (sig - not sure if you would classify this as a pronoun) which also exists in German (sich - his/her/it/them-self) and behaves the same way in that the only other forms are myself (mich) and yourself (dich).
Is it correct to say Jag tvätta mitt barn. and Jag tvätta mig.? Thanks.
If you change the verb to the present form: Jag tvättar mitt barn. and Jag tvättar mig.