A number of English verbs with a word-final t double the t when a suffix is attached to the stem. However, this is not always the case. Some verbs do not double the t.
Verbs such as hit, sit, forget and regret double the t with the addition of a suffix. This can be seen in forms such as hitting, sitting, forgetting and regretting. The last syllable of the base is stressed. This is the penultimate syllable of the word.
In verbs such as visit, benefit and exit, the t remains single when a suffix is attached. This can be seen in forms such as visiting, benefiting and exiting. The difference between these verbs and the ones with a double t is that with the verbs of this group the stress falls on the first syllable. It does not precede the suffix. The t is only doubled when the final syllable of the base is stressed.
The rule for the doubling of the t has an exception. Bases with a doubled vowel such as oo and combinations such as ea and ou do not double the t. Consider the forms looting, heating and shouting.
The final t of verbs sometimes doubles when a suffix is attached. The rule for doubling is determined by stress. The t can only be doubled if the final syllable of the base verb is stressed.