When is it necessary to use 来 to convey the meaning "in order to" In Mandarin, if we have two VO in order the second one indicates the purpose of the first action, something like "in order to" or just "to" in English, as in:


  我回家学习中文 - I'll go back home (in order) TO study Chinese.


But I've seen many times a 来 appearing to convey that role, as in:


  有些作业老师让学生拿回家来做 - There are some homeworks the teacher let the students take back home TO do.


So, my question is:
Can I always use 来 when I want to convey that meaning or does it have some specific cases?

Thanks
Simple rule: If you've got another verb after 他， translate 来 as to.

Of course, things are a bit more complex! 


把杯子拿到这里来。(来 corresponds with hither)
把杯子拿到那里去。(来 corresponds with hence)
他用水来浇花。(来 corresponds with to)
他用铅笔来画画 (来 corresponds with to)
他明天来看你。(来 corresponds with ? He tomorrow to see you ?) 
他马上来和我们一起吃饭。(来 corresponds with ? come? to?) 


Nothing ever 'to' but it also 'come', just depends where you're standing!

You can't really use English to make sense of Chinese! Chinese makes sense!