Both sentences follow the pattern: SVO.

In both sentences there's a nominal clause functioning as a direct object.

I know something = I know how beautiful she is
S V O

Any reason the second sentence is past tense?

What's so strange about the use of the past in the second sentence? You mean the use of "Now" with past tense? It may sound strange, but please read the following definition from the cambridge dictionary online:

now adverb (IN SPEECH)

/na?/

used in statements and questions to introduce or give emphasis to what you are saying

Now, where did I put my hat?

I know how beautiful she is.

and

Now I realized how brave they are

It's not 'now' that makes me think the tense is wrong.

I know=present simple

I realized=past simple

Why is there a change in tense?

Wouldn't it be more correct if he knows and realizes?

The speaker is conveying his belief that they are brave by nature -- "permanently" brave, so to speak.

CJ

CalifJimThe speaker is conveying his belief that they are brave by nature -- "permanently" brave, so to speak.

CJ

And how does the past simple do this, while the present simple cannot?

It's the present simple that conveys it.

CJ

CalifJimIt's the present simple that conveys it.

CJ


Is the past simple incorrect then?

Only the present seems right to me, but what do I know.

I think the two of us are talking about a different clause.

The following is correct.

Now I realized [past] how brave they are [present].

The use of the present (second clause) is the speaker's way of telling us that he considers their bravery an essential part of their character -- something they continue to have in present time (even though he realized i

LOL, I thought the two setences in the original post were combined by and, not two individual sentences!

I know how beautiful she is.

and

Now I realized how brave they are.