"I do know that."
It's not in his nature to apologise. It's not in him to say that he regrets being gone before first light, the last time he was here. He doesn't know how.
Other options:
no subject
"I do know that."
It's not in his nature to apologise. It's not in him to say that he regrets being gone before first light, the last time he was here. He doesn't know how.