What is the combining vowel?

In grammar, a combining vowel is a vowel sound that is inserted between the root and the suffix of a word. This vowel is usually used to join two words together, or to make the pronunciation of a word easier. For example, in the word "beautiful", the combining vowel "i" is used to join the root word "beauty" with the suffix "ful". Combining vowels can also be used to change the part of speech of a word. For example, the word "run" can be changed into a noun, "runner", by adding the suffix "er". In this case, the combining vowel "e" is used to join the root word with the suffix.