why did it say it was wrong in the first part
I'm not sure which test you're referring to but I imagine you're talking about this question you got wrong, when asking for "un" hermano.
Tengo __ hermano.
I have a brother.
The correct answer is "un", not "uno". For "a ..." before a masculine noun you need "un" as explained in the lesson.
I hope it clarified it.
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