Is retrograde planet strong or weak?

it depends on the ascendant, if ascendant functional malefics are being retrograde it will give good results, but negative results in that bhava matters

if ascendant functional benefics are being retrograde it will give bad results and also results negative in bhava & yoga matters too

in ep36 i have tried to explain the same

9dtdI82zZTY

https://www.youtube.com/watch?v=9dtdI82zZTY

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for example, take leo ascendant if mars and Saturn are being retrograde in this case mars is functional benefic to the ascendant. some times related to 4th and 9th house far from mother, away from motherland, litigation in the purchase of lad, breaks in education, less support from the father..etc may come negative and also dasha may not work.

Saturn retrogression is functional malefic so it will give extremely good results related to 7th house , but 6th & 7th bhava will get failed. but Saturn works like exalted Saturn
7th house yoga good results: extremely good fame in society, good management skills. dasha will work positively
6th & 7th house bhava negative results: failure in marriage, health problems..etc

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you need to observe carefully about consideration and cancellation of retrogression when rahu and ketu are connected and also retrograde planets need to come to direct motion within the 30 days from the date of birth.