那些频率副词可放在句首?

sometimes,seldom,usually,often,frequently,rarely,never,.有一道题是what do you do on weekends?回答为什么是sometimes；；；；；；；；；因为不是每个周末都做同样的事情呀。即“有时候。。。。。有时候。。。。” 明白？还有那些题需要注意？seldom, , rarely, never 等表否定以上的频率副词如果放在句首，要用【部分倒装语序】如：Never have I seen him before.=I have never seen him before.Seldom does he go there.=He seldom goes there.