Hi. I would like to pay D by bitcoin. It's my first time sending that sort of money. Can anyone explain me how he would know that I paid him? Since I have only adress of his account bitcoin, no name or surname I would for example send money and then what? How he would know that I paid, not someone else?
He would know because he generated that number just for you and will probably only use it to receive payment from you. With bitcoin/crypto/block chain transactions you can use a new number every time you make a transaction unlike a fiat/bank transaction where you have a single account number.
It is important when using bitcoin, to triple check everything. Unlike a bank transfer there is no way to retrieve any money sent to an incorrect address. There's no customer service line or central hub you can email to chase any faulty payments. You probably already read this when learning how to set up your wallet and purchase crypto coins but it doesn't hurt to reinforce that nees for diligence. :)
Another thing worth thinking about is how quickly you want your payment to go through. Your choice of transaction fee teir will influence this. On some wallets you set it manually and other have a 3 teir system of low - medium - high. The blockchain is kept ticking over by miners that earn a fee from having their equipment make the transactions occur. They will choose the transaction with the most return first as there are many miners fighting for those fees so transactions don't happen on a first come first served basis. This means that the time it takes for transactions with low fees attached to them can take quite some time to get picked up. Days, even weeks if you're really unlucky or particularly tight lol. After a certain amount of time your payment will bounce back if it doesn't get picked up by a miner but it can become "stuck" and this can stop you making further purchases from your wallet until it becomes unstuck. There are some tricks to getting a transacrion unstuck but you don't want to find yourself in that situation. Choose a high fee (its still cheaper than bank transaction fees) and triple check any details you have to manually input and this should help your bitcoin transaction go smoothly.
Rest assured, D or whoever you send payment to will know the transaction came from you. You can also prove it to them if you were to ever need to because the block chain is publicly recorded and accessible so its easy to show that a payment from your wallet address was sent to the number they provided. You won't need to do it (so don't worry) but it is possible.