Complete the sentences below.
nouns ending in y preceded by a vowel form their plurals by adding . -s/-ies
most nouns form their plurals by adding . -s/-ies
nouns ending in s, ss, sh, ch, x, or z form their plural by adding . -s/-ies
nouns ending in o preceded by a vowel form their plurals by adding . -s/-ies

When changing a noun ending in y preceded by a vowel to its plural form, we must add -s. That's what we can notice in the following examples: boy - boys; toy - toys; day - days.

As for the majority of nouns, we add -s to make their plural form. Notice the examples: computer - computers; phone - phones; blanket - blankets; pencil - pencils.

Nouns ending in s, ss, sh, ch, x, or z form their plural by adding -es (this option was not included in the question). As we can see in the following examples: watch - watches; fish - fishes (not a very common plural, but it does exist); mix - mixes; mass - masses.

Nouns ending in o preceded by a vowel need -s to form their plurals. As the following examples may show: radio - radios; stereo - stereos; video - videos; cameo - cameos.

faith or loyalty when pledged in a solemn agreement or undertaking.