Old nodes will never accept the new chain if they only accept blocks of up to 1 MB and there is a block of 1.2 MB in the competing chain, they will always reject the chain because its not in accordance to their consensus. 
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That makes it clear, thank you!
Blockquote
Thank you for your help
I completed the section of forks and i have a question . Like i see it a hard fork can continue to exist forever ? Or does it stop when 100% of the Network updates ? I do know that soft forks Will be removed because the original chain Will grow faster and therefore continue to be the only valid chain . Just wondering what is the case for a hard fork ? @alko89
Until there is someone willing to run the updated (or not updated) node that contains the hard fork, there will exist a network with that fork. Like in case of Bitcoin Cash for example
So you have an accidental fork which consists of âtwoâ blocks with the same block height. How can two blocks have the same information in them? If each block had the same information then there would be no need to get 5 or 6 verifications.
This was question number 1 in the quiz.
The blocks are not the same. They were mined at the same time but contain different data and were mined with a different nonce but are both valid because they have a hash that is below the target value.
I understand that, however, the quiz answer to question one said the blocks where the same and that they did not have different information which, I believe, was choice number 3.
Was is the correct answer?
How does accidental forks happen?
Your Answer
check_box Miners mine the same block at the same block height but with different contents
Correct Answer
check_box Different miners find a block at the same block height at the same time.
âMinersâ vs âDifferent minersâ. I think it is the english translation. Miners is multiple miners where different miners is multiple miners. Option one is âminers: mine the same block at the same block height but with different contents. I guess the right answer is " Different minersâ find âaâ block vs the âsameâ block. However, it is not worded very will because the first option contains the words " different contents" where as the ârightâ answer says a block but should have stated âdifferent miners find a block which each block mined contains different informationâ.
Yes, but if the block has different content its not the same so the correct answer holds.
When a hard fork occur, would that mean all the stale block starting from the genesis block become vaild??
There is a set block height when the new rules take effect. All blocks up to that point are treated by the old rules.
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exactly my question 
Thanks a lot!
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If a soft fork update occurs, and you have less than 50% of the mining power. Would that mean that soft fork that happen is now a hard fork within the network??
Technically it would mean that the softfork was rejected and would probably be reverted. Because both blocks are valid according to old and new nodes the chain with most PoW would win in this case.
So even if some blocks get mined with the new rules the longest chain rule would eventually drop them.
When you have an accidental fork, the transactions in the disregarded block(s) (stale blocks) get sent back to the mempool. In the video it is said that you can loose your money if you donât wait for at least 6 confirmations which would contradict the fact that stale block transactions get sent back to the mempool to be mined again for approval. What is true?
Not sure what is contradictory here. The more confirmations you wait (6 confirmations is just a rule of thumb) less likely it is that your transaction will be part of a stale block because more blocks have been appended on the said block.
More blocks you add, the harder it is for the block to change. One reason for this is because its harder to make a 51% attack on older blocks because more blocks have to be remined and in case of an accidental fork because with more blocks it is less likely two miners will mine again a block on each of their chains again at the same time.
Theoretically two miners can again find a block at the same time and append it to their chains but with more blocks this becomes less and less probable. After 6 confirmations the probability of this occurring is close to 0 and you can be fairly certain your block wonât get dropped.
This is all understood but it was said in the video of lesson 3 that you loose your money if your transaction becomes part of a stale block which would contradict the fact that the transactions get sent back to the mempool to be revalidated and mined again. Which would mean that you donât loose your money but your transaction just takes longer to settle.
I think question 1 in this quiz needs to be revised
How does accidental forks happen?
Your Answer
check_box Miners mine the same block at the same block height but with different contents
Correct Answer
check_box Different miners find a block at the same block height at the same time.
Ad Answer 1: You cannot mine the same block at the same time with different content because then it would not be the same block. I thought the question meant to mean: mine the next sequential block in the block chain each with different content, which would be the correct answer.
Ad Answer 2: This âcorrectâ answer suggests that two miners find one block at the same blockheight at the same time so if this is the correct answer it should read at the end ⌠with different contents.
A bit confusing. Other questions very clear.