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Is there any rhyme or reason to when one should double the last consonant when adding -ed or -ing?
It has always been a word that intuitively I wish to spell with two Ts. So does anyone know why it's not writting?
Could it just be that the T is harder when said in a continuous tense while the T is harder, for example, in the word bite than in its continuous form?
You spell it with one ‹t› because if it were spelt with two, it would rhyme with hitting instead of with fighting.
As for how “hard” your ‹t› is, compare these:
[ˈɹʷʌɪɾɪŋ][ˈɹʷɪʔn̩][ˈɹʷaɪɾɪŋ][ˈɹʷɪɾn̩][ˈtʰʌɪʔn̩][ˈfoʊˌtʰɑn]Notice how only the last one has a “real” ‹t› in the middle of it.
Edit: Integrating some comments.
You’re right that the ‹t› in writing is different from the one in written. The one in writing is still heard, although it is often a simple flap: [ɾ].
However, the ‹t› in written often reduces to a mere glottal stop: [ʔ]. That means it is not heard; the glottis just stops moving for a moment.
Phonemic /t/ in English has several different allophones, which vary by word and sometimes by speaker. Expect to hear /t/ realized as any of [tʰ], [t], [ɾ], and [ʔ], depending on various complex factors.
[ɾ] as it is in the middle of writing or in the word cara in Spanish, Italian, or Portuguese, but rather as you say it is often simply “omitted”, here represented by using a glottal stop [ʔ] as I have written it above. Note that per your question, there is no analogy in English with the difference between Spanish pero and Spanish perro, where the first is a simple flap /ɾ/ and the second a fully rolled /r/. Also, those are phonemically distinct in Spanish.
