mein
my
Possessive article of the 1st person singular. It expresses that something belongs to the speaker or is related to them.
Ich habe mein Portemonnaie zu Hause vergessen.
I forgot my wallet at home.

my
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my
Possessive article of the 1st person singular. It expresses that something belongs to the speaker or is related to them.
Ich habe mein Portemonnaie zu Hause vergessen.
I forgot my wallet at home.

The possessive article 'mein' is declined in the singular just like the indefinite article 'ein'. So if you know the endings of 'ein', 'eine', 'einen', 'einem', etc., you can also use 'mein' correctly.
Hier ist ein Stift. Hier ist mein Stift.
Here is a pen. Here is my pen.
'Mein' can also stand alone as a pronoun and then replaces a noun. In this case, it gets a strong ending that indicates gender and case.
Wessen Tasche ist das? – Das ist meine.
Whose bag is that? – That's mine.
'Mein' expresses not only possession (mein Haus (my house), mein Buch (my book)), but also personal relationships (meine Familie (my family), mein Freund (my friend)) or belonging to abstract things (meine Idee (my idea), meine Meinung (my opinion)).
Mein Bruder wohnt in einer anderen Stadt.
My brother lives in another city.
to be
'sein' and 'mein' look similar, but they mean something completely different. 'sein' is a verb and describes who or how someone is, where something is, or serves as an auxiliary verb. 'mein' is a possessive article and shows that something belongs to me or is connected to me. The words can be confused quickly when reading because only the first letter is different. Remember: 'mein' is usually followed by a noun, for example 'mein Buch' (my book). 'sein', on the other hand, is a verb or part of a verb form.

Im Garten habe ich mein Fahrrad neben den Baum gestellt.
In the garden, I put my bicycle next to the tree.
Im Zug sind wir vor dem Termin ziemlich ruhig.
On the train, we are fairly calm before the appointment.