Idete yuku michi shiramaseba arakajime imo o todomemu seki mo okamashi o

出でて行く道知らませばあらかじめ
妹を留めむ関も置かましを

Roma-ji: Idete yuku michi shiramaseba arakajime imo o todomemu seki mo okamashi o
English Translation: Had I only known
the road she would take away,
beforehand, surely,
to hold my dear one from going
I'd have set a barrier there.
Meaning: Had I known the road by which she would leave, I would have set up a barrier beforehand to keep her from going.
Commentary: Book Three Elegies 468

Headnote continued from the preceding poem: "Another poem with tanka composed by Yakamochi" — "Envoy"
This is the second envoy to the long poem mourning his wife. Had he known beforehand the road by which she would set out on the journey of death, he would have set up a barrier there to hold her back—frustration before a death that could not be helped. Likening death to a departure and imagining a barrier to stop her conveys his desperate wish not to lose her.
This belongs to a sequence of elegies that Ōtomo no Yakamochi composed from the sixth month of summer into autumn in Tenpyō 11 (739), mourning his late concubine.
The poet, Ōtomo no Yakamochi, was a son of Ōtomo no Tabito and a Nara-period official. He served as Governor of Etchū and of Inaba, among other posts, rose to Middle Counselor, and died in Enryaku 4 (785). He is thought to have been deeply involved in compiling the Man'yōshū.

First line "the road she would take away" refers to the journey of death.
Second line "Had I only known": mase, the stem of the counterfactual mashi, with ba gives "if I had known."
Third line "beforehand, surely" means in advance.
Fourth line "to hold my dear one from going" means to keep his wife back.
Fifth line "I'd have set a barrier there": seki is a checkpoint, mashi counterfactual, and o exclamatory.
Author: Otomo no Yakamochi
Source: Manyoshu
Prev Poem: Toki wa shimo itsu mo aramu o kokoroitaku iyuku wagimo ka midoriko o okite
Next Poem: Imo ga mishi yado ni hana saki toki wa henu waga naku namida imada hinaku ni