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

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.

Answer

The present perfect is used for unfinished or undefined time. I don't think your example is correct, since for a year is understood to be a finished time. A time period of one year. It would have been correct if you wrote:

I have walked downtown every day this year.

In that case you'd have unfinished time. The time period would be up to and including now. If you used a defined and finished time in the past like last august, then you'd have to use the simple past.

I walked downtown every day last August.

Since last August is finished time, you have to use the simple past.