Your mouth is made up of many muscles. Like all muscles, those you use when speaking need training and regular exercise to develop your ability to make new sounds and sound patterns and to create unfamiliar words with ease.
With time and appropriate practice, your mouth and speech muscles (and memory muscle!) will develop new capabilities to match your new language.
Phonemes are the sounds of a language, or the sounds of letters and letter combinations. Speakers combine the sounds to make words. In English, there are 26 letters (the alphabet) and 44 sounds.
It is important to know the names of letters (the alphabet), to understand that different sounds exist and how to make them to create words (for clear understanding), and to recognize the relationship between spelling and sounds (also for understanding).
An example: cat
Cat is spelled c-a-t.
The three letters in cat have three distinct sounds that sound differently than their alphabet names. It is important to know how to pronounce the three sounds together to create the word in speech, and it is also important to know how to spell the word aloud if needed.
The Alphabet: American Pronunciation for Adults by English with Morgan
26 Letter Sounds by Spencer @ Toddlers Can Read
44 Phonemes by the Rollins Center for Language & Literacy
Pronunciation exercise for BUSY PEOPLE [5 minutes]
Short Vowels (æ, ɛ, ɪ, ɑ, ʌ, ə, ʊ) | 44 Sounds of American English
Learn All English Sounds & Pronounce Words Perfectly
Pronunciation Rules by Rong-Chang
Pronunciation Exercises: Minimal Pairs by Rong-Chang
Pronunciation and listening practice using word pairs with similar (but different!) sounds
"Every language sounds different and uses different sounds. Different muscles are employed. The mouth, cheeks, nose, and tongue, along with breathing in or out, may be combined in almost endless ways."
~ Gregorio Billikopf Encina, University of California