I have a lot of problem using has, have and had. It confuses me very much. What is the rule of using these three words, considering all the possible situations?
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.'
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.'
