But why come up river to these open sand flats?但為何要逆流而上到這片開闊的沙灘來呢

come up升起；靠近，走進

His new book did not come up to expectation.

他的新作沒有達到預期水平。

You've come up in the world, haven't you?

你的地位已經上升了,是嗎?

But this nursery will not exist for long.但這個托兒所無法長期存在

exist有時有“在逆境中生活，僅能活命”的意思。

Some people in some poor countries receive wages that allow them to exist but not to live.

有些貧困國家的人民的收入只能勉強維持生計。

exist與subsist區別：subsist指“繼續存在”，而exist指“存在”。

While they wait for it to grow,they do their best to learn the skimming technique.等待下喙長長的同時他們會努力學習捕食技巧

wait for 等候； 等待

It irks us to wait for people who are late.

等候遲到的人使我們厭煩。

He was then sitting on thorns and waiting for you.

他當時如坐針氈，焦急地等待著你。