If bitcoin is splitted after August,and someone send the btc to some other btc address during this period, will those in trasit btc lost for ever?
You can take ETH and ETC for example, both have similar addresses but if you send ETH to ETC address you will not get any ETC there but you can use private key of that ETC address (if you have) to move ETH out of it.
It is advised to not move any coin during first few days of august, you might actually lost your bitcoin at the time of splitting or segwit activation. Read more here
https://bitcointalk.org/index.php?topic=2017191.0