He sat (all) by himself, reading the newspaper.他独自坐着读报。

The new law will help, but it can't solve the problem (all) by itself.新法会有帮助，但不可能仅靠它解决这个问题。

You shouldn't lift that heavy box (all) by yourself. Let me help you.你不应该一个人提那么重的箱子。我来帮你。