The rules above will help you, plus you need to think about what you hear. For example, you would not say, "gooses". What do we say? "I see the geese crossing the street." With words ending in a consonant and -y, you drop the -y and add -ies, like city is cities. But if it ends in a vowel (a, e, i, o, u) with a y, you need to keep the y and just simply add an -s, like donkeys. Lastly, those ending in -f, you will need to drop the -f, and add -ves, like calf becomes calves.