"She eats chicken but does not eat fish."
Translation:Ella come pollo pero no come pescado.
"Pero" is for expressing contrast: Estoy cansada, pero terminaré la tarea. I am tired, but I will finish the homework. "Sino" introduces an affirmative idea in the second clause, which has been preceded by a negative idea in the first clause: No peleó Pedro sino Juan. Pedro did not fight, Juan did.
Right out of my "Spanish Sentence Builder" book.
Another good explanation here: http://spanish.about.com/od/conjunctions/a/sino_pero.htm
Sino is an elegant way to provide a non-obvious alternative to a negative statement, without having to start a new sentence.
To avoid confusing it with pero, try replacing it with although–if it works, it's pero; otherwise, it's sino.
> — Me apetecía verte hoy, pero hasta mañana no puedo.
> "I felt like seeing you today, (but/although) until tomorrow, I can't."
> — Pues yo no contaba con verte hoy, sino mañana, así que perfecto.
> "(Well) I wasn't counting on seeing you today, (but/I was counting on seeing you) tomorrow, so that's perfect."
I go deep into the usage context of sino in this NachoTime post.
You have asked a really good question, oppenheimj. I remember reading something about this a while back, but for the life of me, I could not remember where I had seen it. Though initially I had thought that either "pescado" or "pescados" should be acceptable here, I began to wonder if Spanish makes a distinction between the singular and plural as we do in English. For example, in English, we would say,
"She does not eat fish."
If we were to say instead,
"She does not eat fishes,"
it sounds odd. Typically when the plural of "fish" is used, we are using an idiomatic expression or referring to more than one species of fish. For example:
"He sleeps with the fishes now."
"Global warming is having a negative impact on native fishes."
I wonder if such usage is similar in Spanish. Though I have posted the question at Spanish StackExchange, I do not have an answer for you right now. If I should receive a reply that answers your question well, I'll post it here. Or, if you prefer, you can follow what discussion might develop by clicking on the link below:
Final Note: The question has been answered and you can view it by clicking on the link above.
In standard English, the comma is unnecessary in your sentence because you do not have two independent clauses. "Does not eat fish," does not express a complete thought. As it is shown on screen, however, I would say there are two independent clauses because the Spanish verb includes the subject. So... why no comma? I don't know.
Your post reminded me of something I wrote a while back in response to a comment made in a forum outside of duolingo. Without any additional explanation, I think it addresses this topic fairly directly, so I'll just paste it below:
I am familiar with the rule about adding a comma before a conjunction in a compound sentence, but, at least in American English, the rule is optional when the independent clauses of the compound sentence are short. So that you don't have to take my word for it, I have included an excerpt from a book titled, The Best Punctuation Book, Period: A Comprehensive Guide for Every Writer, Editor, Student, and Businessperson:
"When independent clauses joined by a conjunction are short and clear, the writer has the option of not using a comma."
Examples given are as follows:
"Jane likes pizza and she also likes pasta."
"You could stay or you could go."
"I walked there but I ran home."
"Vegetables are packed with vitamins and that's important."
"Pack your things and go."