be

almost
His response was almost instant.

brief, fleeting
For a brief instant, I thought she was going to fall.

one, single
His news was too important to be contained for a single instant longer.

pause for
He paused for an instant before continuing.

for an ~
Just for an instant I thought he was going to refuse.

in an ~
It was all over in an instant.

~ after/before
She woke up in the instant before the phone rang.

(any) given, (any) one
At any given instant the distribution of molecular speeds is always constant under the same conditions.

very
He took out his keys to lock the door. At that very instant the door flew open and a man ran into the room.

at … ~
The bomb could go off at any instant.