I am an Android developer and I try to import a wallet from its private key, everything goes well but there will no mnemonic code for this wallet, and I know private key comes from mnemonic code too. I only just wonder isn't possible to generate mnemonic code from the private key, I am new to Ethereum and need help. Thanks in advance.
Private key corresponds to the an Ethereum address and public key. The mnemonic is, in general, used to derive a tree of public-private key pairs for BIP39 format. Therefore, the private key to mnemonic conversion is technically, not possible. Otherwise it is a serious security issue.
For more information on how this generation work and what is a deterministic wallet, refer this answer here.