Native Korean speakers often struggle with the English "ch" [as in church] and "j" sounds due to differences in voicing and aspiration, as Korean lacks a strict voicing distinction and uses more aspirated consonants.
To produce the "ch" sound (as in "cheese"): [voiceless]
Place the tip of your tongue against the ridge behind your upper teeth.
Build up a bit of air pressure.
Release the air by quickly pulling your tongue back, creating a "ch" sound.
To produce the "j" sound (as in "jump"): [voiced]
Place the tip of your tongue against the ridge behind your upper teeth.
Build up air pressure, similar to the "ch" sound.
Release the air more gently, while also using your vocal cords to create a voiced "j" sound.
Both sounds involve the tongue and airflow, but "j" is voiced (vocal cords vibrate), while "ch" is voiceless.