thank you for your post.
I'm sure this is something other people have wondered about as well.
I will try to explain it:
Subject case: "mai" means "I" and is used in sentences like "I am" for example "Mai Nikita HooAO" (I am Nikita)
Objective case: "mujhe" means "to me/ for me" and is used in sentences like "I want", "I think", "Give me" for example "Mujhe chaay chahiye" (I want tea) or "Mujhe lagathaa hai baarish hogee" (I think it will rain) or "Mujhe dheejiye" (Give me)
Posessive case: "mere" means "my" and is used in sentences like "my pen", "I have a pen" for example "meraa kalam" (My pen) or "Mere paas ek kalam hai" (I have a pen)
Here the words mujhe and mere are formed when different case forms are added to the noun maiM.
1.Objective Case (karm kaarak)
The objective case in Hindi is obtained by adding ko to noun.
maiM + ko = mujhko or mujhe
He gave me a pen
Us ne mujhe kalam dhee OR us ne mujhko klalam dhee
Here the word mujhe is used to give the meaning to me
Take another sentence
I think it will rain
mujhe lagthaa hai baarish hogee
Here , even if the English sentence starts with I, the corresponding Hindi sentence starts with mujhe which means to me. In Hindi, expressions like I want or I think are expressed using the objective case form i.e., mujhe.
2.Possessive case (sambanDh kaarak)
Possessive case is obtained by adding kaa, ke or kee to noun.
maiM + kaa = meraa ( when the object related is singular masculine)
Eg: My son = meraa betaa
maiM + kee = meree ( when the object related is singular / plural feminine )
My daughter = meree betee
My daughters= meree betiyaaAO
maiM + ke = mere ( when the object related is plural masculine / or in expressions like I have)
My sons = mere bête
I have a pen =
Now , take the example I have a pen. This is also a possessive case sentence. So the corresponding Hindi sentence is mere paas kalam hai
The whole expression I have is substituted with mere paas. Here the word mere does not mean I , but it is the possessive case form.
I hope that sheds some light on it. Please let me know if you have further questions.
All the best for your Hindi learning.