they are forced back down to Earth.它们不得不回到地面上

be forced to被迫(不得不)

I was forced to sign the agreement against my will.

我被迫违心地在协议上签了字。

The truck was forced to lie by for repairs.

卡车不得不停下来修理。

as the lake is prone to flooding.因为湖水随时会泛滥

be prone to易于 ...

Youths are prone to easy temptations.

年轻人容易被引诱。

Some people are prone to jump to hasty conclusions.

有些人往往作出轻率的结论。

This is when it's at its most vulnerable -totally reliant on its parents.雏鸟这时候最为脆弱完全仰赖父母照顾

rely on依靠；信赖(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－－他光会说空话。

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

既然你长大了，就不应该依靠你的父母。