1. sometimes常可用于句首。如：

Sometimes she comes late. 有时她来得晚。

Sometimes she didn’t agree with me. 有时她和我意见不一致。

Sometimes we get a lot of rain in August. 有时在8月份雨水很大。

2. often用于句首时，通常表示强调，且其前一般有quite, very修饰。如：

Very often he comes in late. 他常常迟到。

Quite often the phone rings when I’m in the bath. 电话经常在我洗澡时响。

3. usually有时也用于句首，其前不用修饰语。如：

Usually I get up early. 我平时起得早。

Sometimes he comes by bus, but usually he comes by taxi. 有时他坐公共汽车来，不过他通常还是打的来。

Usually cooking pots have two small handles but pans have one long handle. 通常地深底煮锅有两只把手而平底锅只有一个长长的把手。

4. always一般不用于句首，除非是用在祈使句中。如：

Always remember this. 请时刻记住这一点。

Always look in the mirror before starting to drive. 一定要先看看反光镜再开车。

另外，never也可用于祈使句的句首。如(from www.yygrammar.com)：

Never tell him the news. 千万不要告诉他这消息。

Never ask her about her marriage. 决不要问她结婚的事。

5. 在正式文体中，表示否定意义的副词seldom, never可位于句首，但此时其后要用倒装语序。如：

Seldom has there been such a happy meeting. 过去很少有过这样愉快的会议。

Never did he think the book would be finished so soon. 他可没想到这本书会这么快看完。