Aki-san has already explaind to you and I'd just like to write what I would say with "started/fineshed/was eating". 僕がつく前に、ベンジャミンはもう(すでに)食べ始めていた。 ＝Before I arrived, Benjamin had already started eating. 僕がついたとき、ベンジャミンはもう(すでに)食べて始めていた。 ＝ When I arrived, Benjamin had already started eating. 僕がついたとき、ベンジャミンはもう(すでに)食べ終わっていた。 ＝ When I arrived, Benjamin had already finished eating. 僕がついたとき、ベンジャミンはもう(すでに)食べていた / 食べているところだった。 = When I arrived Benjamin was (already) eating. I hope this was helpful.

this is a challanging and interesting question. Let me write what i know for now. I will try to find answer also for myself. 1st things 1st. (In my opinion) Japanese language has no sense of 'tense (= past, present, future)', but has only that of 'aspect (done/completed/fixed/confirmed or not,) '. ～（し）た：　「た」　comes from 「たり」、an old japanese particle meaning 'complete ( in aspect)'. And I believe you already know it (...to be continued)