Monophthongs are vowel sounds that have a single, unchanging quality throughout their duration. Unlike diphthongs, which combine two vowel sounds in one syllable, monophthongs maintain a consistent sound. Examples of monophthongs in the English language include the sounds represented by the letters "a" in "cat" and "e" in "bed." These vowel sounds can be categorized based on their position in the mouth, such as front, central, or back. Each category can further be divided into tense and lax vowels. Understanding monophthongs is essential for studying phonetics and improving pronunciation in languages like English.