catch it与get it的例句

catch it -
This is the last bus today. You'd better catch it, or you'll never get home by tonight.
这是今天的末班车。你最好赶上，否则今晚就会不了家了。

You speak so fast. I can't catch it.
你说得太快了，我跟不上。

get it -
What do you mean? I don't get it.
你是什么意思？我不明白。

Please don't be hesitate to ask, if you don't get it.
如果你不明白，请不要犹豫问（我）。

不知道你是从哪里查到的，但是“跟上”的正确表述应该是catch up.正如你说过的，catch up & get it 都是口语化的表达，要用书面的语句来造句还真是挺囧的。随便想一个情景给你吧：
u r slowing down, do catch up or u might get lost! u get me?

wait,wait,wait,I can't catch it!
Ok,got it!

“Robert, tell Mr Charles that the meeting is at 12 o'clock"
"Got it."

catch it 得你说的那种用法很少被使用，而且，catch it在口语中还有挨骂的意思。虽然语境不同是不难区别，仍建议少用。