Should a man who committed adultery and then proposed to a girl tell her about his past, or should he keep it a secret? And does his not telling her affect the validity of the marriage?

One must hasten to sincere repentance and beware of anything that might lead back to temptation. It is known that a reformed state after repentance is a sign of its acceptance.

Along with repentance, it is necessary to conceal the sin and not inform anyone about it, because it is obligatory for a Muslim to conceal their own faults, in accordance with the Prophet's (peace and blessings be upon him) saying: "All of my Ummah will be forgiven except for those who make their sins public."

If the conditions for the validity of marriage, as mentioned in Fatwa No. 30897, are met, then the marriage is valid, and informing them about the sin does not affect it. However, one should not inform them of it.