Hi Tung Quoc
If you place an adverb of time (now) at the beginning of the sentence, it is more emphatic:
Now I understand everything.
I understand everything
now.
Both are correct.
It is usually not a good idea to put anything between the main verb (understand) and its object (everything), and word order in English is very fixed. That's why the main verb is usually before its object, in other words,
understand must come before
everything.Things are different in some other languages. There is a joke about two people listening to someone delivering a speech in German. One of the two didn't understand German at all, so after they had listened to the speech for a minute he asked his German-speaking friend: "What's he saying?"
"I don't know yet. We're waiting for the verb."
Cheers
CB