The phrase 'not completely understand' is correct and usable in written English.
You can use it when something has been explained to you but you haven't quite grasped it in its entirety. An example sentence might be "Although the teacher gave an excellent explanation, I'm afraid I do not completely understand the concept."
Like the storage retailer, it's building a new market that consumers may not completely understand.
Do say: "The executive chairman of Saatchi and Saatchi might not completely understand women".
I admit, I do not completely understand all of the bills.
At this point, researchers admit that they do not completely understand how the deep learning networks make decisions.
Though scientists do not completely understand what triggers solar flares, Stanford solar physicists Monica Bobra and Sebastien Couvidat have automated the analysis of those gigantic explosions.
And yet while this Doctor might not completely understand these human impulses, it doesn't even touch the sides of his loyalty towards her.
The scientific community does not completely understand what causes hangovers, Dr. Preston R. Miller, a surgeon at Wake Forest, said in an email.
They are also obviously highly successful – and part of that success must be due to their socialisation, held together by bonds we still do not completely understand.
These methods have the potential to lead to the development of robots that can adapt to uncharacterized environments and which may be able to perform tasks that human designers do not completely understand.
The Newark local's president, Richard E. McDonaugh Jr., said those who voted against the contract did not completely understand its details, a theme also raised by other union officials.
Joseph Burke, the company's chief financial officer, said that the company had little long-term debt and did not "completely understand" the rationale of Standard & Poor's decision last week to downgrade the company to junk bond status.
