"Peter works hard."

Translation: पीटर मेहनत करता है ।

[Maya925906]

Why is it not पीटर मेहनत काम करता है ?

[vinay92]

'मेहनत करना' itself means working hard.

You can say पीटर मेहनत का काम करता है but that has a different meaning - Peter does work that is hard.

[OJFord]

Can you say मेहनत ‍ से ‍ काम कारता है though? Or does that only work in the imperative? (I'm pretty sure I remember मेहनत से काम करो from that lesson.)

[vinay92]

You can but there is a subtle difference. You would use मेहनत से काम करो/ पीटर मेहनत ‍से काम कारता है when you have a specific काम in mind. Along the lines of 'Put effort into the work'.
On the other hand, मेहनत करो/पीटर मेहनत करता है is used when you are talking about working hard in general.