/ɑ/ is the most released and lowered back vowel sound.
A challenge with the /ɑ/ sound is that the speaker might not open the mouth enough...
...so a word like "Hop" closes off to sound like "Hope".
So let's practice by starting with a jaw release here.
Follow along with me.
Open your mouth and take your knuckles and massage your jaw muscles for about five counts.
One, two, three, four, five.
Good! Now, let's keep one palm on the side of your cheek, like this, and I want you to notice the difference between /ɑ/ and /oʊ/.
Can you see how the jaw is more open for /ɑ/?
Let's try comparing some /ɑ/ and /oʊ/ words.
And now, a few sentences.
John likes Joan.
He has a clock in his cloak.
That's not my note.
Keep your practice up on EnglishCentral and good luck.