The phrase "actually understood that" is correct and usable in written English.
You can use it when you want to emphasize that someone has comprehended a particular point or concept, often in contrast to previous misunderstandings.
Example: "After our discussion, I finally actually understood that the project deadline was moved up by two weeks."
Alternatives: "truly grasped that" or "genuinely comprehended that".
"Who actually understood that country, Syria, best - was it us, was it other people, or was it Assad himself?
That may seem an oxymoron when referring to a vehicle wearing the wreath and crest (and harboring 12-foot tailfins as part of its history), but CTS designers actually understood that slapping wood and chrome all over the place was not the answer it's not what BMW and Mercedes do, after all.
And Aaron actually understood that, and he took it in good spirits.
I don't know that I actually understood that I was saying that we're all just afraid of each other.
But I imagine that, for someone with a job that ninety-nine per cent of people don't actually understand, that might be additionally stressful.
Like a weight had been lifted that I didn't actually understand that was there, which was a very interesting part of it, that you don't understand what you're walking around with.
"That you could actually understand that victim's history and where they might be at any given period of time; what are the cities they've been in over time; What are the various phone numbers they've used over time?
I even actually understand that Soboroff was simply using the new tools of our old trade when he interviewed Maher.
And the truth is that the KKK and all the other white supremacists groups in the United States and Europe who use the confederate flag actually understand that flag better than the Sons of Confederate Veterans.
I'm shocked daily when I realize how few people actually understand that gay marriage in New York, or any other state doesn't mean that our problem is solved.
While their science teacher's metaphor is useful for explaining the concept to the audience, the only way that the kids themselves actually understand that place is by attaching it to their pre-existing understanding of the Vale of Shadows, an analogue for the actual Dungeons & Dragons Plane of Shadow.
