Was and were we are using for past continous tense...

[Ulan]

Dear teachers,

Was and were we are using for past continous tense. But som times with was we are using past participle of the verb. Why? and where we will use did and had with past participle form of verb(past perfect tense)?

Many thanks
Ulan

[Mister Micawber]

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'Was + pp' is the past simple passive: 'I was sent by the department'.
'Had + pp' is past perfect: 'I had seen the letter already'.
(X) 'Did + pp' is not a correct English structure.
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Native English teacher at Mister Micawber's