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<p>“I have been eating” = I started eating time ago and am still eating</p>
<p>“I had been eating” = I started eating time ago and was still eating when happened. “I had been eating” is always followed or preceded by a second action, as in “I had been eating for an hour when he came in” or “when they arrived I had been eating for an hour.” </p>
<p>“I will have been eating” = I started eating or will start eating sometime and will continue eating until happens. You’ll almost never see these constructions.</p>