Why is there "Change" in Bitcoins? I have read about a change address and wondered why there even is the concept of change?
Let's say I received 1 BTC from some person A and I want to send 0.4 BTC to another person B, afterwards. From what I have understood, I have to send the 1 BTC, as a whole and then get 0.6 BTC as change.
Is that right? And if so, why don't you just send 0.4 BTC in the beginning?
Bitcoin follows a Unspent Transaction Output (utxo) model.
Our modern banking systems follow an account model - you're assigned a bank account number (similar to an address), and send and receive money from it. Any incoming funds increase your total account balance, and any outgoing funds decrease it. The bank does not keep track of which funds came from which deposit. It's just a single balance.
Bitcoin is closer to cash - Any funds in your Bitcoin wallet are the result of a previous incoming transaction, exactly how any notes in your physical wallet are the result of previous real life transactions you've made (whether an ATM/bank withdrawal, or received as change at a store).
Bitcoin tracks the output of each individual transaction, just like real life notes. When you receive 1 BTC, you receive an output (note) of exactly 1 BTC. This must be spend in its entirety, and cannot be split. So if you'd like to send 0.4 BTC, you put in the entire 1 BTC into the transaction, and send 0.6 BTC to one of your own addresses in the outputs.
This works much the same way as buying a $5 item with a $20 note - you must put in the entire $20 note, and you will get $15 back. You can't simply tear the $20 note into four equal pieces and create 4 $5 notes.
Of course, unlike real cash, in Bitcoin there is no risk of the recipient simply taking your BTC and refusing to return the change - the change is sent back to an address owned by you as part of the transaction building process. The recipient of the rest of the funds has no control over it.
A cleaner way to think about it is using the coin (physical coins, not bitcoin) analogy mentioned by Abdussamad in the comments - You can treat each output in your wallet as a single, indivisible coin of a fixed value.
When you spend these coins, transactions act as a melting pot - you can add in multiple coins to create a pot with a value of the sum of all coins, and you can use the melted coins to mint new ones of the values you want - 1 coin for the payment you're making, and one coin for the change that you keep with you. This even accounts for transaction fees - every time you melt down some coins, the pot claims a small amount that sticks to it, resulting in a final output that is slightly lower than the input.