To say that something is mine you say mo or m’, followed by the thing that you are talking about: mo pheansail.

---

To say that something is yours, you say do or d’, followed by the thing that you are talking about: do pheann.

peann >>> a pen
mo pheann >>> my pen

àireamhair >>> a calculator
m’ àireamhair my calculator

rùilear >>> a ruler
mo rùilear >>> my ruler

You will see that mo is shortened to m’ before a vowel or vowel sound.

If the noun following begins with an f, mo is shortened to m’ and an h is added to the noun:
m’ fhòn-làimhe.

Mo and do can change the word that comes after them. This is called lenition.

Try these when you want to say in my, in your etc:
na + mo = nam bhaga in my bag
na + mo = nam bheachd in my opinion
na + a = na bhaga in his bag
na + a = na baga in her bag