A strong planet strengthens the house it owns and weak planet weakens the owned house.
Planet aspecting its own house strengthen it. If Planet aspecting its one of the two sign, than its another sign is also strengthened. Ex: Jupiter if aspects pisces than its another sign sagitarius also gets strength.
Irrespective of the planet being natural malefic or benefic, it can be malefic or benefic according to ownership of house in horoscope. Ex: In cancer lagna, Mars being natural malefic it becomes a functional benefic planet in this lagna due to lord of kendra and trikona.
Lords of the 6th, 8th and 12th houses are considered malefic.
Lords of quadrant (kendra) and trine (trikona) are benefic.
House gets weak if its lord is placed in any trik house i.e., 6th, 8th or 12th.
House gets weak if it has lord of any trik house is placed in it, i.e., lord of 6th, 8th and 12th weakens the house where it sits.
House become strong if its lord is placed in kendra or trikona.
Planet placed in lagna becomes strong.
A planet if becomes lord of both kendra and trine is said to be Yogkarka, a higly beneficial planet which always do good.
If natural benefic planet owns two kendra houses it losses its benefic nature.