我们去不了有两个原因：第一，我们很忙；第二，路途太远。

There are two reasons we can't go: ___firstly, we're busy, and secondly, it's too far.

我认为这是个好主意，首先是因为它能节省时间，其次是因为它更便宜。

I think it's a good idea, ___firstly because it saves time, and secondly because it's cheaper.

要提高你的语言技能，首先，试着每天阅读。其次，尽可能多地练习口语。

To improve your language skills, ___firstly, try to read every day. Secondly, practice speaking as much as possible.