The suffixes -ful and -less are word endings used to form adjectives, often with opposing meanings. When added to a root word, -ful signifies "full of," while -less signifies "without". 

Suffix: -ful

• Meaning: Full of a particular quality.
• Forms an adjective: It can be added to nouns and verbs to describe someone or something as having a quality in abundance.
• Spelling rule for root words ending in 'y': When adding -ful to a root word ending in a consonant followed by 'y', change the 'y' to an 'i'.
• Examples:
  • beautiful (from beauty + -ful): Full of beauty.
  • helpful (from help + -ful): Full of help.
  • painful (from pain + -ful): Full of pain.
  • thoughtful (from thought + -ful): Full of thought or consideration. 

Suffix: -less

• Meaning: Without a particular quality.
• Forms an adjective: It creates a new word describing someone or something as lacking a particular quality.
• Spelling rule for root words ending in 'y': As with -ful, if the root word ends in a consonant plus 'y', change the 'y' to an 'i'.
• Examples:
  • harmless (from harm + -less): Without harm.
  • painless (from pain + -less): Without pain.
  • thoughtless (from thought + -less): Without thought or consideration.
  • useless (from use + -less): Without use. 

Comparison: Antonyms

Since -ful and -less have opposite meanings, they are often used to create antonyms from the same root word.