Suffix -IVE is a working E suffix. Suffix -IVE changes the base word to a state of “doing or being”, as in “fest” to “festive”.
When a word ends in Suffix -IVE, the syllable before the suffix is most often accented. 

Sometimes, an extra syllable is added before Suffix -IVE, as in “compete” to “competitive”. 

When an extra syllable appears before -IVE, or the base word already has its accent on the first syllable, the accent may stay on or move to the first syllable. Try that accent pattern when the usual pattern does not produce a familiar word. 

Suffix /ĭv/, Suffix /ĭv/, Suffix /ĭv/, festive