It depends on the context, you could have either...
I reached an understanding with Tommy not to fight any more.
Tommy and I reached an understanding to stop fighting.
more understanding, most understanding
A homophone for "had an understanding of" is "had understanding of."
my wife is very Understanding. Are you understanding the situation?
The adjective of understanding is understandable.The adverb of understanding is understandably.
There are two acceptable plurals - memorandums of understanding and memoranda of understanding.
There are many animals that represent understanding. An alligator represents understanding deceit, and a crab represents understanding the power of dance.
Memorandum of Understanding (MOU)
what has our understanding of electromagnetism led to?
Understanding has four syllables.
Understanding why countries celebrate special ceremonies.
a understanding of right and wrong
They had a mutual understanding of the terms and conditions.