قَدْ comes immediately before the past-tense verb.
Without Qad: وَصَلَ الْقِطَارُ
The train arrived.
With Qad: قَدْ وَصَلَ الْقِطَارُ
The train has arrived.
The subject remains the subject and the verb remains past tense. قَدْ changes the shade of meaning; it does not swap grammatical roles.
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قَدْ comes immediately before the past-tense verb.
Without Qad: وَصَلَ الْقِطَارُ
The train arrived.
With Qad: قَدْ وَصَلَ الْقِطَارُ
The train has arrived.
The subject remains the subject and the verb remains past tense. قَدْ changes the shade of meaning; it does not swap grammatical roles.