how do i know when to use meu and when to use minha?
Everything has a gender in Portuguese. Meu=for male Minha=for female. Normally the nouns ending whit an A will be Female. Minha garrafA. Meu casaco.
I was about to ask that. Thanks for answer!
It also accepted "É minha garrafa." Which is the more common usage?
Both are equally common. The "a" is not stressed anyway, so in everyday speech you might not even hear it if someone speaks fast enough.
in a grammar test.. are both acceptable?
Yes. You would add "a" if you wanted to emphasize that you are talking about your very own bottle.
The program didn´t like "É a garrafa minha". Is this not customary?
No, you would need the verb between the noum and the possessive, like in: a garrafa é minha
Could it be "Issa é a minha garrafa"?
No. Issa is not a word in portuguese. It could be ISSO. Since, roughly saying IT=ISSO. But in this case ISSO would be used in a ironic/depreciative way. It is hard to explain writing
Thank you. The Meu vs. Minha explanation has helped me a lot