Consider this sentence:

> 
> du spinnst
> 
> 
> 

It means you're crazy. Now, where did the 'are' come from? One may assume from this example that the 'are' comes with the verb but in other cases we have to explicitly write out 'are' using conjugations of sein (bin, bist etc). Eg:

> 
> Du bist müde
> 
> 
> 

Question: When do we need to explicitly add a *sein* and when do we not need to ?