In Italian, the meaning(s) conveyed by this sentence can only be written one way: "Io mangio". But in English, "Io mangio" can be translated as either "I eat" or "I am eating". (I'm having difficulty choosing the best words to explain this.) In English, "I eat a bean" does not completely express the meaning, while in Italian "Io mangio un fagiolo" is a complete thought. To convey the same meaning, the sentence would have to be translated as "I am eating a bean". In English, if we say "I eat a bean" it feels like it is not a complete sentence, even though there is a subject and a verb. The grammatical structure of one language is not always comparable to the grammatical structure of another language. This is one of those situations where they don't compare exactly. In English, we use the present continuous more often than we use the simple present.