When you DO need a combining vowel:
* To prevent awkward consonant clusters: For example, "sun" + "rise" = "sunrise" (without the "i", it would be "sunrse", which is difficult to say).
* To maintain the correct pronunciation of the root words: "hand" + "book" = "handbook" (the "o" helps keep the "hand" sound).
* When the root words end in a consonant and begin with a vowel: "air" + "plane" = "airplane".
* To distinguish between words with similar spellings: "man" + "power" = "manpower" (as opposed to "manpower").
When you DON'T need a combining vowel:
* When the second root word begins with a vowel: "high" + "way" = "highway" (no need to add an extra vowel).
* When the second root word starts with a consonant and the pronunciation is clear without it: "black" + "bird" = "blackbird" (the "b" sound is clear enough).
Remember:
* Language is constantly evolving, and there are exceptions to every rule.
* Dictionaries are your best friend! If you're unsure, consult a dictionary to see how the word is spelled.
Ultimately, using a combining vowel is about making the word easier to read and pronounce. While it's not always strictly necessary, it's often a good idea to err on the side of clarity.