きみがもどって来るころには、ぼくは出かけてしまっているよ。

By the time you come back, I'll be gone.

きみがもどって来(く)るころには、ぼくは出(で)かけてしまっているよ。

Kimi ga modottekuru koroni wa, boku wa dekaketeshimatteiru yo.

君 (きみ)

you; buddy; pal; monarch; ruler; sovereign; (one's) master

が (が)

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

戻る (もどる)

to turn back (e.g. half-way); to return; to go back; to recover (e.g. something lost); to be returned; to rebound; to spring back

は (は)

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

僕 (ぼく)

I; me; you; manservant

出かける (でかける)

to go out (e.g. on an excursion or outing); to leave; to depart; to start; to set out; to be about to leave; to be just going out

よ (よ)

indicates certainty, emphasis, contempt, request, etc.; used when calling out to someone; used to catch one's breath or get someone's attention; yo!

Readings: ライ、タイ、く.る、きた.る、きた.す、き.たす、き.たる、き、こ

Meanings: come, due, next, cause, become

Readings: シュツ、スイ、で.る、 -で、だ.す、 -だ.す、い.でる、い.だす

Meanings: exit, leave, go out, come out, put out, protrude