they are forced back down to Earth.它們不得不回到地面上

be forced to被迫(不得不)

I was forced to sign the agreement against my will.

我被迫違心地在協(xié)議上簽了字。

The truck was forced to lie by for repairs.

卡車不得不停下來修理。

as the lake is prone to flooding.因為湖水隨時會泛濫

be prone to易于 ...

Youths are prone to easy temptations.

年輕人容易被引誘。

Some people are prone to jump to hasty conclusions.

有些人往往作出輕率的結論。

This is when it's at its most vulnerable -totally reliant on its parents.雛鳥這時候最為脆弱完全仰賴父母照顧

rely on依靠；信賴(reliant是rely的adj.形式)

Don't rely on him to do anything he's just a talker.

什么事都不要依靠他－－他光會說空話。

Now that you are grown up, you should not rely on your parents.

既然你長大了，就不應該依靠你的父母。