Bip39 tool - BIP32 Extended Private Key vs BIP32 Root Key In the bip39 mnemonic tool there is three sections :

Mnemonic : we input our words or a BIP39 seed and the tool compute for us the BIP32 Root Key
Derivation Path: there is a BIP32 Extended Private/Public Key
Derived Addresses

Questions :
1- If i want to use Electrum wallet and I have already a wallet. Which key should I have to import ? What is the difference if I import the BIP32 Root Key or if I import the BIP32 Extended Private Key ?
2- Should I use BIP44 instead of BIP32 to import my wallet in an Electrum one ?
If you want to import a wallet, you should import a BIP32 extended private key. It is often derived from mnemonics.

What is the difference if I import the BIP32 Root Key or if I import the BIP32 Extended Private Key ?

The terminology is a bit confusing. The BIP32 derivation is done using a number, called the chaincode, over a private key (called the master private key). This private key has nothing special: it's just the starting point of the derivation.
The concatenation of a private key and the chaincode is called an extended key.
To import your wallet you need to import the master extended private key.
Regarding your question about BIP44, it is not a replacement to BIP32 just a set of recommendations of how to use it.
NB: you might be interested in the description of the key derivation process by the bitcoin.org documentation.