Here are the grammatical errors and other issues in your text:
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"Heung Bu is the brother." - This sentence is incomplete/contextually unclear. It may be redundant since it's already stated that Heung Bu is the younger brother.
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"The main characters in Don’t Count Your Chickens are Mengano and Fulano, the two older brothers, and Juancho, the youngest brother." - This is grammatically correct, but the placement of the commas can be confusing; consider a slight rephrasing for clarity, though this is more stylistic than a grammatical error.
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"The young brother in The Swallow and the Pumpkinseed learned to always be kind and helpful and never greedy." - The phrase "to always be" can be more neatly expressed as "always to be," though this is a stylistic choice.
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"In Don’t Count Your Chickens Juancho learns about patience and never to expect something before it happens." - A comma is needed after "Don’t Count Your Chickens."
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"never to expect something before it happens." - The phrase could be clearer with a slight rephrase, such as "and never expects something before it happens," for parallel structure. However, this is more about style than grammar.
Overall, while the text is mostly clear, there are a couple of areas where it could be improved for readability and clarity.