条件・譲歩N1

いざ〜となると、いざ〜となれば、いざ〜となったら

once it comes to... / when it actually comes down to...

Structure

いざ + 動詞辞書形 + となると／となれば／となったら

Once it really comes to the action, one finds it difficult. The main clause is usually an unexpected fact or difficulty.

いざ話し合いとなると、お互いの意見がなかなか合わない。

Once it actually comes to discussing, our opinions just don't align.

いざ書こうとなれば、何を書いていいのか分からなくなる。

When it actually comes to writing, I don't know what to write.

いざ means when it really comes to...; the main clause is usually about difficulty or failure.