Do not use combining vowel between a and root word.?

It seems you are asking about a specific rule in combining words, but it's not quite clear what you're looking for. It's not a standard rule in English to avoid combining vowels in any specific way.

Could you please clarify your request? Are you thinking of:

* Specific words: Are you referring to a particular group of words where you're trying to avoid combining vowels? (e.g., "pre- + announce" vs. "pronounce")

* Word formation: Are you asking about a general rule for forming new words by combining parts?

* Specific grammatical context: Is there a specific grammatical situation you're trying to avoid combining vowels in?

Please provide more details so I can understand your request better and give you a helpful answer!