phosphorus(noun) and phosphorous(adjective). Can you explain me with some sentences when to use these forms.
Not much to explain, Guest-- you have said it already: 'phosphorus' is a noun and 'phosphorous' is an adjective. So when you are writing about the chemical element, you use the noun, as in 'Phosphorus has an atomic number of 15 and an atomic weight of 30.973761'; and when you are writing about something which is phosphorus-like, you use the adjective, as in 'Phosphorous acid is handy for making phosphites'. A confusion arises, however, in that 'phosphorous' is also an alternative spelling for 'phosphorus'.

Good luck in all your endeavors.
And, although both may be pronounced the same, the noun is "PHOSphorus", and the adjective is most often "phosPHORous" (like "phosPHORic"). At least that's how my chemistry teacher said them.