No rule. The most accurate translatino for "walk" is "caminar", and "andar" is an alternative translation. "Andar" is a tricky verb, because it could have different meanings. You can use this sentence as an example. Here, "anda" may mean that he's in a relationship with my sister, or that he walks with my sister, or just that he's with my sister (he's going somewhere accompanied by my sister, not necessarily walking).
I've always understood that you should use: "Caminar" for walking in order to get someplace specific (i.e., traveling by foot -- it's related to "camino", literally a "path", figuratively a "journey"); "Pasear" for walking around to see and be seen by your community (I think there's also another word for this, something related to our English "promenade"...?); "Dar un paseo" when you are just walking around for no particular purpose (lit., 'to give a walk', equivalent to English 'to take a walk'); "Andar" can refer to any and all forms of walking, literal or figurative.
But "andar" might be considered kind of archaic or quaint (at least in rural Panama where I first picked up Spanish -- anyone know if this is true in other Spanish-speaking regions?).
Right, I'm aware of that and I don't see how using a word that has two (rated G) meanings can be embarrassing, especially since everyone will be able to deduce what you're trying to say. I thought there was something else associated with this specific sentence that I wasn't aware of.
Thanks for the quick reply jfgordy.
This is accepted now. July 2018.
I think andar/caminar are pretty general and don't have romantic overtones in Spanish any more, or less, than in English. "He goes with her" may or may not mean dating. "He goes with her to school" or "He goes with her to Vegas for the weekend." I think the notion of dating is more often expressed with "salir con" in Spanish.
Yes "andar con alguien" also means to date someone, to go out on a date with someone or to be in a relationship with someone. So, «él anda con mi hermana» fits that very well but unfortunately that meaning is not accepted by Duolingo. I've already suggested that but I am not sure if they will add it or not.
BTW, I learned about this meaning of the verb andar from a Mexican podcast so it could be regional use.
Update (May 9, 2020): Apparently 3 years passed since my original post. Now, Duolingo accepts the following translation in line with what I said above:
He is going out with my sister.
Andar does have the second sense of walking out. We have or had the same expression in English. 'He is walking out with my sister' clearly means that they are in a relationship. So although these two verbs are synonymous in one context, there is that added connotation with 'andar' that 'caminar' doesn't have.
There was a Saturday Night Live skit ('Lothar of the Hill People') that was basically a talk show in caveman times. They used 'walking' as a euphemism for sex. ('I don't walk with my mate as much as I used to.' 'Sometimes I walk by myself'), so making the connection between 'walking' and 'dating' doesn't seem all that unusual for me! Thank you, comedy skits.