Complex sentence: Mandelstam was said to have had an affair with the poet Anna Akhmatova, although Akhmatova insisted throughout her life that their relationship had always been a very deep friendship, rather than a sexual affair.
Split into simpler sentences: Mandelstam was said to have had an affair with the poet Anna Akhmatova. She insisted throughout her life that their relationship had always been a very deep friendship, rather than a sexual affair.