Anyone who is a real English grammar expert?

[Itwasthenandstillis]

I need help with the use of 'but' in a sentence (teaching English as a foreign language).

I don't think either sentence is incorrect grammatically. It is more of a style issue.

Learned and learnt are both correct in UK English. Lots of words have -ed and -t endings (leaned, burned, smelled).

I would say....however, we know more about space....

And

Just leave out the but in the second sentence and put in a comma.

The but is very clunky and unnecessary

In the first sentence, the "but" is perfectly okay, but the second "that" shouldn't follow the "but".
In the second sentence, again the "but" is perfectly fine, but the "it" shouldn't follow, because the structures should be parallel.