Learn Turkish: Bu işin önemi büyük. - The importance of this work is great.

This creates a possessive noun phrase, similar to saying 'the work's importance' in English. The first word gets the owner suffix (-in makes işin), and the second word gets the owned suffix (-i makes önemi).

Where is the verb for 'is' in this sentence?

In Turkish, there is no separate verb word for 'is' in this type of simple sentence. You just place the adjective at the end. So büyük acts as 'is great' all by itself. You might sometimes see a suffix like -tür added to make it büyüktür, but it is usually left out in everyday speech.

Does the word bu (this) describe the work or the importance?

It describes the work. Bu iş means 'this work'. Because it comes before the possessive phrase, bu işin önemi translates to 'the importance of this work'.

I thought büyük meant 'big'. Why is it translated as 'great' here?

While büyük is the standard word for physical size (big or large), it is also used for abstract concepts. When talking about importance, success, or problems, büyük translates naturally to 'great' or 'huge' in English.