The past perfect tense is formed using had before the past participle form of the verb.

Scientists and military personnel had used the Internet before it caught the attention of the public.

As 2001 ended, Microsoft's Internet Explorer had taken the largest portion of the Web browser's market.

In these sentences, two past actions are shown. When two actions occur in the past, the earlier action takes the past perfect form of the verb and the later action is in the simple past tense.

Time expressions usually start with when or before followed by a verb in the simple past tense.

When the sentence is in the passive voice, had been is used before the past participle of the verb.

The Internet had been used by scientists and military personnel before it caught the attention of the public.