The –ent or –ence ending is often used if a word has a soft c, a soft g or a qu sound, or when a related word has a clear e sound near the end.
soft c: Innocent innocence
soft g: intelligence intelligent
Follow this link for some work sheets to help you learn these spellings: (33 onwards)
I will give you 6 root words to learn. You need to be able to use them with all suffixes as I will randomly pick on the day.