INSTANT: an adjective meaning immediate or without preparation.
INSTANCE: a noun meaning an example or a particular case.
While we are using the words in these forms and senses, there is little confusion:
MY INSTANT RESPONSE IS NEGATIVE.
IN THIS INSTANCE, LITTLE HELP IS POSSIBLE.
However, INSTANT can also be a noun, meaning a precise and limited moment of time.
THE INSTANT AFTER YOU TAKE CONTROL WILL BE A LITTLE FRIGHTENING.
Both nouns can have plural forms, INSTANTS and INSTANCES. Now we are approaching the danger area.
THE INSTANTS AFTER YOU TAKE CONTROL WILL BE A LITTLE FRIGHTENING.
THESE INSTANCES OF SUPPORT ARE ENCOURAGING.
It is common to become a little uncertain of which word is required in these contexts. Sometimes, people become so confused when talking that they double-pluralize INSTANT: INSTANTSES (good grief!)
Some solutions and road maps:
1. Remember that INSTANT means a precise moment and INSTANCE means an example, and quickly assess the meaning before choosing the correct word.
2. (The safe, if mildly cowardly, pathway.) Abolish INSTANT – or at least INSTANTS – and INSTANCE from your vocabulary and replace them with MOMENT(S) and EXAMPLE.