Here's why:
* "Cer" has a long "e" sound, similar to the "e" in "see".
* "tain" has a short "a" sound.
Therefore, "certainly" is not a short vowel word.
Here's why:
* "Cer" has a long "e" sound, similar to the "e" in "see".
* "tain" has a short "a" sound.
Therefore, "certainly" is not a short vowel word.