Had is verb that is used in the past tense, expresses a type of action (or lack thereof) that was completed prior to the moment of speaking or writing.
It is most often used in tandem with a past participle, such as 'had gone' or 'had taken,' and can be combined with other modifiers, such as in 'had nearly finished.' It is generally used with the helping verb 'have,' in a structure such as 'had had.'
1. I had already gone to bed by the time my parents arrived home.
2. She had nearly finished painting the bedroom before she ran out of paint.
3. We had been thinking about taking a vacation for months before a friend suggested it.
When using 'had' in the past tense, it is important to remember to also include another verb to complete the thought. It is helpful to pay attention to the tense of the verb following 'had.' If it is in the present tense, it may indicate a verb tense error. 'Had had' should never occur in the same sentence. It usually indicates redundancy.'