I have three questions to ask:
She was surprised at his boldness and directness.
Can I use 'bravery' instead of 'boldness'? If I can't, could you please explain why?
Hahaha...well you could but it would depend on the context of the sentence.
Context #1 "he" is a knight in shining armour and she is his lady friend and she marvels at his skill in battle. Then yes, "bravery" could be substituted for "boldness"
Context #1 "he" is a man who is trying to make friends with her. She is usually not approachable (perhaps she is rich and he is poor) but this does not stop him. He walks right up to her and states his intentions to become her boy friend. He is being "bold" and "direct". Is he being "brave"? I don't know....perhaps in this day and age he is; perhaps she has a reputation for making such men look like fools as she doesn't like this sort of advance.
But unless she is fearsome, I don't think brave really applies....although in my case, under such circumstances I would not have the courage to do such a thing
We must have these government officals know they cannot abuse their power.
Could you please explain if I can use the word 'understand' instead of 'know'?
"Understand" is a suitable substitute for "know".
He wanted to learn some useful skills.
Can I use 'acquire' instead of 'learn'?
Yes, "acquire" is a good replacement word for "know" here.
Looking forward to hearing from you.
Thank you in advance.