Let’s look at those words again:
put – putting,
big-bigger,
stop-stopped,
fat- fatten,
run – running,
thin – thinner,
sit – sitting,
get – getting,
stop-stopping,
hot-hotter,
swim – swimming…
Do you know why we double up the last consonant?
Say these words to yourself – put/sit/run/swim/thin/get – notice they all have one syllable sounds
Notice they all have one consonant at the end of the word.
Notice they all have one vowel next to the consonant:
vowel + consonant
we usually double the end consonant when we add the following vowel suffixes (-ing,-ed,-er, -est, -en, -ish, -ery, -y)
put – putting, big-bigger, stop-stopped, fat- fatten, run – running, thin – thinner, sit – sitting, get – getting, stop-stopping, hot-hotter, swim – swimming…
So words of one syllable ending in one vowel + one consonant double the last letter before a vowel suffix.
How to correctly spell words you don't know 