The word regret ends with a single T.
In the word "regret," you do not double the final "t." It is spelled with only one "t."
regret
No word begins with tt.
when conjugating the various tenses for the verb regret an extra t is added when adding an ending that begin with a vowel, e.g.He regretted doing itI bet he's regretting his mistake now
The double consonant in the word "start" is the letter "t."
It comes from a similar French word and in French the final t would not be proounced.
When suffixes such as -ed and -ing are added to words with the accent on the final syllable, they may double a single consonant at the end (this rule has several exceptions). In some cases, it avoids confusion with another word that drops a final E (hopping/hoping). T words that do this include admitting permitting quitting remitting submitting
The double "t" in "quitting" follows the spelling rule for adding a suffix to a word ending in a short vowel followed by a consonant. By doubling the consonant, we preserve the short vowel sound in the base word "quit."
the words letter, better, setter... and probably some more
bookkeeper
It is a matter of ennunciation. The word can be broken into two distinct parts: "stut" and "ter". Notice how you can hear two subsequent t's when you say the word aloud. Since it has two t sounds, it is spelled with two t's.
One word that has double Es, As, and Ts is the verb "devastate".
The letter "e" at the end of a French word usually signals that the final consonant should be pronounced.