When used as a main verb, has/have indicates the present tense and had indicates past tense. Compare:
'I have a very bad cold, so I can't come camping this weekend.'
'I had a very bad cold last week, but I'm feeling better now.'
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Students often make mistakes, I find, when combining the past perfect with the past simple. The past perfect form of the verb always contains had and a past participle and in using the past perfect with the past simple you are indicating that one action happened before another action in the past. Compare:
'When we arrived at the party, we found that the food had all been eaten.'
'We still hadn't finished the exam, when the invigilator told us to stop writing.'
'After I'd had my lunch, I sent all my e-mails.'