"Kept", "cop", "cup", "cope", "keep", "cape", "coop", "cat":
When you say these words, do any of them sound the same or very similar? Unfortunately, that is not a good sign!
One of the key differences between American English and other languages is that, while other languages focus heavily on consonant sounds like b, c, d, f, g..., American English heavily focuses on vowels.
It's way if you say "still" like "steel", your listener may look at you confused, though they sound the same to you! The reality is that to native American English speakers, those sounds are completely different and cannot be changed.
You may be wondering then about how to start hearing the difference and pronouncing it differently (in fact, that's one of my favorite benefits of studying pronunciation--you will notice that your listening will actually improve first as your ear starts adjusting to noticing the difference between sounds. This is called "noticing": you cannot easily pronounce a sound that you cannot hear, so it is often the first step).
One of the best exercises to do to improve your American English vowel sounds (or any sounds you find confusing), is to study minimal pairs! Minimal pairs are groups of sounds that are similar and may be commonly confused.
"Still" and "steel" are minimal pairs in English.
"Head" and "had".
"Low" and "law".
"Past" and "pest".
Do you say any of these the same way? Maybe you even say all of them the same! But again, for American English native speakers, these will all sound different.
To get started working on your American English vowel sounds with minimal pairs, here is a video to get you started! Go slowly, identify the most difficult sounds, and start targeting those weaknesses! https://youtu.be/RSuXVzEfYNE