Uno becomes a pronoun when it replaces a noun. E.g. ¿Tienes hermanos? Tengo solamente uno. But thinking about it as a pronoun won't help you in this case because it isn't a pronoun even though it means "one" and not "a". The rule for uno is that when it appears before a singular masc. noun you drop the o. You follow the same rule for ninguno (which literally means "not one"). Tengo ningun hermano. Feminine nouns remain una, ninguna. It's also the same for larger numbers like "Tengo veintiun hermanos". But when answering the question "¿Cuántos hermanos tienes? You can say "Tengo veintiuno."
Does where the "only" comes in the sentence make a difference to the meaning? When I read "I only have one brother." is seems to me that I have only one sibling, a brother. When I read "I have only one brother." is can be that I have other siblings, but just the one brother. Does anyone else read them differently?
In fact, this is the more grammatically correct answer as it is not good practice to split up the verb from the subject - eg. 'I only ate one apple' could imply that you are excusing your action - i.e. I only ate one, I didn't throw it at someone! Subtle but different. Google split infinitives if you want a better answer
Yes helen.pope you are accurate even though you got a down vote for some reason. I thought that the idea was to learn languages, including English. If Duo did the dreaded split infinitive then it seems prudent to point that out. "Only" is one of the most insidious misplaced modifiers in written English. Thanks...
You're right that "only" is often a misplaced modifier. "She only eats apples...." (but does not grow them, for example) is different from "She eats only apples...." (meaning and nothing else.)
It's impossible to split an infinitive in Spanish, but when it's done in English nobody even notices. Take the most famous split infinitive of them all: "To boldly go where no man has gone before." BTW, split infinitives are not the grammar bugaboo once thought. We do it all the time.