When is the present perfect tense used instead of the past tense? 

I know that the present perfect tense is used when some adverbs (e.g., *never*, *ever*) are present in the sentence; the same is true for sentences like the following one.

> 
> When you returned, I have been at home since 3:00 PM.
> 
> 
> 

In which other cases should I use the present perfect? Do the following sentences require it?

> 
> I have walked downtown everyday for a year.
> 
> 
> 

> 
> I have been at home since 3:00 PM.
> 
> 
>