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I know French fairly well and pronouns are mandatory. However, in Spanish, pronouns are dropped quite often. Is there a rule of thumb for this? E.g. are pronouns only used for emphasis? Do they distinguish between formal and informal language?
Subject pronouns are indeed used for emphasis so in your example you want to emphasise that it is you who eat. They are also used, especially in the third person, if otherwise it would be ambiguous whether it is he, she, or you formal who is the subject. Object pronouns are usally not dropped. If you are learning as a native speaker of English you would probably do better to drop when in doubt as using the subject pronoun unnecessarily will sound odd though everyone will understand you.