Animals:
* ant
* ape
* armadillo
* alligator
* antelope
* aardvark
Body Parts:
* arm
* ankle
* abdomen
Buildings:
* apartment
* airport
* auditorium
Concepts:
* air
* ability
* age
* art
* advice
* agreement
Food:
* apple
* avocado
* apricot
* asparagus
Furniture:
* armchair
* armoire
Nature:
* air
* atmosphere
* autumn
* aquifer
Objects:
* airplane
* anchor
* arrow
* axe
* alarm
* address
Places:
* avenue
* alley
* airport
Things:
* answer
* action
* advantage
* amount
* appearance
Other:
* accident
* ache
* activity
* angle
* area
* audience
This list is not exhaustive, but it provides a good starting point for understanding common nouns starting with "a".