Nouns:
* day
* dad
* dog
* door
* dust
* drum
* dime
* debt
Verbs:
* do
* dig
* draw
* drop
* drive
* dare
* deal
Adjectives:
* dry
* dark
* deep
* dead
Adverbs:
* down
*
Pronouns:
*
Conjunctions:
*
Interjections:
* darn
* duh
This is not an exhaustive list, but it covers a good range of one-syllable words starting with "d."