Tā de mǔqīn zài tā hěn xiǎo de shíhou jiù guòshì le.
Her mother passed away when she was very young.
Tips
register
过世 is a polite, respectful euphemism for dying. More formal than 死 (sǐ, to die) and similar in register to 去世 (qùshì). Appropriate for speaking about elders or in formal contexts.