"primary form" is a correct and usable phrase in written English.
You can use it when referring to the original or earliest form of something, for example, "The primary form of communication is verbal."
They are the primary form of navigation.
Looking to buy or sell Polyvinyl alcohols in primary form?
The primary form of funding is the research grant.
HOMES are the primary form of wealth for most Americans.
China is one of the few countries using coal as its primary form of energy.
Even when couples aren't traveling, though, e-mail has become a primary form of communication.
The Legislature is considering two bills that would make injection the state's primary form of execution.
When in San Francisco, I embrace exercise as my primary form of socialization.
A primary form outlining the minimal requirements for participation could serve as an initiation.
They used abstract design as a primary form and ornament as an end in itself.
According to DHS officials, a foreign passport is an acceptable primary form of ID for travel.
