The Suffix Rules
Double a single consonant after a single simple stressed vowel before adding a suffix beginning with a vowel. Hence 'hopping', 'beginner', 'stepped', 'forgotten', 'referral' etc.
Drop a silent 'e' before adding a suffix beginning with a vowel, including 'y' (unless the 'e' is needed to keep a 'c' or 'g' soft). So we get 'hoping', 'smiled' (the 'e' is not the original silent 'e' but part of the suffix), 'operator', 'smoky' etc., but 'outrageous' and 'serviceable' (the 'e' stays in to keep the 'g' and 'c' soft). 'wholly', 'duly', 'truly', 'awful' and 'argument' are exceptions.
Swop the 'y' at the end of a base word for an 'i' before adding any suffix at all unless there is a vowel before the 'y' or the suffix itself begins with 'i'. Hence 'marriage', 'carried', 'reliable' etc. but 'conveyor', 'displayed', 'enjoyment' (vowel before the 'y') and 'carrying', copyist' (suffix begins with 'i').

Simple (short) vowel sounds: /a/ /e/ /i/ /o/ /u/                        

Complex (long) vowel sounds: /a-e/ /ee/ /i-e/ /o-e/ /u-e/ /oo/…...

Vowel suffix examples: -ing -ed -er -able -or -y -ous…...

Non-vowel suffix examples: -ment -ly  -less…...

Ask the child to underline the vowel/s and state whether it is simple or complex, then follow the rules (see above) to add a vowel or a non-vowel suffix.