-ence vs -ent

[kandalf]

I always forget when I should use '-ence' and when I should use another one.

We celebrate Independence day.
There have been a lot of improvement.
He need to take the placement test.
My correspondent did not pick up his phone.

[Nowitzki]

Yes, suffix is sucks

As far as I remember,
-ence is usually for noun (see: http://dictionary.reference.com/browse/-ence)
-ent is usually [originally] for adjectives, but later on also appear in nouns (see: http://dictionary.reference.com/browse/-ent)

It is hard to differentiate those two since we have to see its Latin origin first.

From your example:

We celebrate Independence day.
There have been a lot of improvement.
He need to take the placement test.
My correspondent did not pick up his phone.

Independence > Noun
Correspondent > The origin of this word is as an adjective (see: http://dictionary.reference.com/browse/correspondent)

meanwhile, Improvement and Placement are words with suffix -ment