what had happened was...

基本上這是past tense...包括had happened的past perfect tense和was...主句的simple past tense.

這句其實是一句告訴別人曾經目睹事件發生的statement，例如告訴警察口供時會講的句法，用這種tense是因為講述/回憶的都是過去見過的事(雖然你講的時間是現在，英文人表達的用句/句法便是注重"時態"的定位，中文人由於中文沒有"時態"作句，只會用附加詞作為"時空"的指標：

What had happened was that he drove the car towards the bus stop and hit five pedestrians who were queuing there.

以上供你提問的句法結構配上實際"口供"的內容，並且是一句多層的複合句子(因為一般形容一件事都由多層的情況組合而成的)，希望是類似你見到的句子。

用中文表達你問的那句式的意思是：

當時(had)發生(happened)的情況(What)是(was)...

You should show us the whole sentence.

What you have written here is an incomplete sentence with at least two clauses.

"What had happened" is a noun clause that serves as the subject of the main clause.

"Was" is the verb of the main clause.

"Had happened" is in past perfect tense.

"Was" is in past tense.

No worries, thanks mate.

冇呢樣嘢，你係邊度抄？