Why didn't the 't' double when changing light to lighten?

The reason the 't' does not double when changing "light" to "lighten" relates to the vowel sound in the base word. In this case, the vowel sound in "light" is a long vowel sound defined by the "igh" spelling, which typically does not lead to consonant doubling when forming related words.

When adding a suffix that begins with a vowel, like "-en," the doubling of the final consonant only occurs if the base word ends in a single stressed vowel followed by a single consonant (e.g., "tap" to "tapping"). Since the final vowel sound in "light" is long, it does not create the same condition for doubling the consonant. Therefore, "light" simply becomes "lighten" without a change in the 't'.

In contrast, words like "hat" form "hatting" with a doubled 't' because the last syllable contains a short vowel sound. This distinction is key to understanding why the spelling remains as "lighten" rather than "lighhten."