Two tenses that are often confused are the present perfect (have + past participle) and the past perfect (had + past participle). These two tenses have completely different uses, and you should understand how to differentiate them.

The present perfect (have + past participle) refers to the period of time from the past until the present.

• Sue has lived in Los Angeles for ten years.

This sentence means that Sue has lived in Los Angeles for ten years up to now. According to this sentence, Sue IS still living in Los Angeles.

Because the present perfect refers to a period of time from the past until the present, it is not correct in a sentence that indicates past only.

• At the start of the nineteenth century, Thomas Jefferson has become* president of the United States.

• Every time Jim worked on his car, he has improved* it.

In the first example, the phrase “at the start of the nineteenth century” indicates that the action of the verb was in the past only, but the verb indicates the period of time from the past until the present. Since this is not logical, the sentence is not correct. The verb in the first example should be the simple past “became”. The second example indicates that Jim “worked” on his car in the past, but he improved it in the period from the past until the present. This idea also is not logical. The verb in the second example should be the simple past “improved”.

The past perfect (had + past participle) refers to a period of time “that started in the past and ended in the past, before something else happened in the past”.

• Sue had lived in Los Angeles for ten years when she moved to San Diego.

This sentence means that Sue lived in Los Angeles for ten years in the past before she moved to San Diego in the past. She no longer lives in Los Angeles.

Because the past perfect begins in the past and ends in the past, it is generally not correct in the same sentence with the present tense.