If we use our first language sounds, rhythms and tones when we speak the words of another language, this causes “accent”. We often do this without realizing it because we learned those sounds and tones while still in our mothers’ wombs—at a certain month of in the pregnancy (sorry, can’t remember which month)—babies can recognize their mother’s first language (when it is spoken outside the womb) and other languages that are spoken. These first language patterns are so instinctive, that by the time we are born, we know a great deal about our first language sounds, especially intonation and rhythm…we just can’t say words yet because our speaking parts haven’t developed sufficiently.
Many languages don’t use the AE sound in the English word ‘cat’. When they say ‘c-a-t’ they pronounce it like another English word ‘cot’. Nearly all languages have the AH sound, but very few have the AE sound. So that becomes part of one’s accent—saying ‘cot’ instead of ‘cat’ for the kitten.
And many languages don’t use the same strategies for marking stress that English does. Our first language stress markers may occur in a fixed place in a sentence rather than be variable, as in English.
If you speak English words and use your first language stress and intonation patterns, you will be very difficult for native speakers to understand—you will have a heavy accent.
Changing your accent is easy to describe and hard work to do. It’s all a matter of looking at your first language and your new language, comparing what’s the same and recognizing what’s different, and then working hard on the different parts. These are unconscious habits a person is trying to change, and some are muscle movement habits that they’ve repeated billions of time in their lives. It takes dedication and practice to change them. But it’s possible. If you have a musical ‘ear’, you may be better at making this change.