ば
(Ba)
once (X happens).
Connection rules
- +ひとたび + V plain + ば / と / たら + main clause.
- +'Once X, Y inevitably' — emphasizes that one instance is sufficient.
- +Often followed by ば, と, or たら.
Examples
1.
ひとたび決めたら、最後までやり遂げる。

Hitotabi kime tara, saigo made yaritogeru.
Once I've decided, I see it through to the end.
「once (X happens)」
2.
ひとたび約束したら、守るべきだ。

Hitotabi yakusoku shi tara, mamoru beki da.
Once you've promised, you should keep it.
「once (X happens)」