Sample this:

The English language had a great impact on the modern culture.

>> impact is not there anymore.

The English language has a great impact on the modern culture.

>> impact is going on right now - it has not been completed.

The English language has had a great impact on the modern culture.

>> impact is completed and we are describing English language (now) as having this quality.

Here "had" is past-tense of "has" so what you are saying is the impact is over but this itself is in present tense for the subject "English language".

Does this makes sense?

Arun

If we replace 'had' with any other past participle word, it will help to understand. for example:

"Dr. Rowan ,whose secretary resigned two weeks ago,has forcedto do all his own typing."

I feel usage of 'had' is simple past participle usage of verb after 'has'.

Please correct me, if wrong.