/ɑː/ is a long vowel sound.
Open your mouth wide.
Your tongue should rest in the bottom of your mouth.
It's pronounced /ɑː/.../ɑː/

It's a farm cart.
/ɪts ə fɑːrm kɑːrt/
I park the car.
/aɪ pɑːrk ðə kɑːr/
Are the stars from Mars?.
/ɑːr ðə stɑːrz frəm mɑːrz/
He carved a large heart in the bark.
/hi cɑːrvd ə lɑːrdʒ hɑːrt ɪn ðə bɑːrk/
The hard part is to start the car
/ðə hɑːrd pɑːrt ɪz tə stɑːrt ðə cɑːr/