en can be translated in many ways, so don't go at it from the English side but from the French. As the corresponding lesson states, "en" as a pronoun replaces "de + object".
J'ai aceté une douzaine d'oeufs. -- I bought a dozen eggs.J'en ai acheté une douzaine. -- I bought a dozen of them.
Je lui parle de son problème. -- I talked with her about her problem.Je lui en parle. -- I talk with her about it.
Pierre m'a offert un morceau du gâteau. -- Pierre offered me a piece of the cake.Pierre m'en a offert un morceau. -- Pierre offered me a piece of it.
-- Chris (not a native speaker).
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