As far as i know and i hope i know well...we use the present perfect in the sentence ''We've often eaten in this restaurant'' because the action has been taking part since a moment in the past, until the moment you say it, in the present. We use the present in the sentence ''We often eat in this restaurant'' because you made a habit from that, it'a something general and it's not neither about the past, either about the present.
Please correct me!! i'm not a teacher...:)
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