What suffix is commonly added to words that end with 'S'?

The suffix commonly added to words that end with 'S' is typically the suffix -ed, especially when forming the past tense of verbs. For example, the verb "pass" becomes "passed" when you add the suffix -ed.

When considering the other suffixes:

• The suffix -tion is often used to form nouns from verbs, such as "inform" to "information," and does not directly relate to words ending in 'S'.

• The suffix -ing is also used to form present participles or gerunds from verbs, such as "pass" to "passing", and again doesn't specifically relate to words ending in 'S' in the same manner as -ed.

• The suffix -ity forms nouns that refer to states or conditions, like "curious" to "curiosity," and this suffix is not typically added to words that end with 'S'.

Therefore, -ed stands out as the suffix that is naturally associated with creating past tense forms from verbs, making it the most suitable choice in relation to words that end with 'S'.