She watched his slow progress down the steep slope.

她望着他慢慢走下陡坡。

显然，down 是介词，介词后面接宾语the steep slope。副词的话，the steep slope 做什么成分呢？

再看这句：

He walked down along the river. 他沿河而下。

down 是副词，无宾语可接。along 是介词。

这里的当down是什么词性？以后遇到这种该如何更好地辨别呢？

She watched his slow progress down the steep slope.她望着他慢慢走下陡坡。

这里down是prep.还是adv.呢? 对于这两个总是分不清?

   【答】如果down是prep.后面必须接宾语；如果down是prep.还是adv.，后面不能接宾语。如：

    She watched his slow progress down the steep slope.她望着他慢慢走下陡坡。（down 是介词，the steep slope 是宾语）

    We watched as the sun went down. 我们看着夕阳西沉。（摘自《牛津高阶英汉双解词典》第9版p.638）此句中的 down 是副词。