You use instant to describe something that happens immediately.
Mr Porter's book was an instant hit.
波特先生的书立刻引起了轰动。
Instant food is food that you can prepare very quickly, for example by just adding water.
He stirred instant coffee into a mug of hot water.
他将速溶咖啡搅入一大杯热水中。
instants
An instant is an extremely short period of time.
moment
For an instant, Barney was tempted to flee.
那一瞬间,巴尼很想逃走。
If you say that something happens at a particular instant, you mean that it happens at exactly the time you have been referring to, and you are usually suggesting that it happens quickly or immediately.
At that instant the museum was plunged into total darkness.
那一刻,博物馆陷入一片黑暗。
the instant
To do something the instant something else happens means to do it immediately.
I bolted the door the instant I saw the bat.
我一看到那只蝙蝠就把门闩上了。