directly involved in the most active or dangerous part of something
直接参与某事中最活跃或最危险的部分 (zhíjiē cānyù mǒu shì zhōng zuì huóyuè huò zuì wēixiǎn de bùfen)
"Doctors were on the front line during the pandemic."
"在疫情期间，医生们在前线。" (Zài yìqíng qījiān, yīshēngmen **zài qiánxiàn**.)
to feel stress or pressure from a difficult situation
to be honest, moral, and respectable
a new environment or situation
to examine a large amount of information or items carefully
to choose to participate
to spread gradually among people
