This is a great example of using a non-cognate to translate a word. Although "conduct" is perfectly correct, "behavior" in general is a more apt word in English, although the context could change that. In my opinion (IMO), "conduct" refers more to following more specific rules, while "behavior" involves generally acting in so as not to violate more general rules. For example, "conduct" describes the way students in a military academy or a parochial school act on campus, while "behavior" describes the way a group of high-school students act while off-campus at a local gathering point. It's a fine point of distinction, but different enough to warrant using "behavior" in this sentence rather than "conduct" to translate "conducta"
i actually hold the opposite view. But like you said, they are both correct. I believe conduct IS the more used in this instance, and probably because it is both the noun and verb, whereas behavior is just the noun. "How does she conduct herself? Well, I am not responsible for her conduct."
http://forum.wordreference.com/threads/responsable-por-de.932147/ --> apparently responsable por is used as a result of the influence of the English language. I was taught in class that it is responsable DE.
Nope the plural "s" of "sus" refers to the noun it's modifying.
Su perro = His/Her/Their/Your dog
Sus perros = His/Her/Their/Your dogs
"Su(s)" is an adjective and (almost) all adjectives in Spanish change from singular to plural along with the noun; for example:
El perro negro = The black dog
Los perros negros = The black dogs
"estoy" is for states where "soy" is for characteristics. In my opinion I think this should be able to be either one, because a parent may refer to it as a characteristic always being responsible for their child, where a sibling may mean it temporarily and it is impossible to determine without context.