How mother tongue can cause difficulties in acquiring English vowels and consonants?

English has 26 letters and 44 distinct sounds. English also contains dipthongs which are two vowel sounds blended together.

A toddler has 50% more synapses in his or her brain than an adult. This allows the child to build neural pathways as a response to the environment. The human brain expands by 300% from birth to adulthood.  So, language acquisition is a large part of what humans do in their childhood.  The language window is open from birth to about age 13. So, learning a new language will be easier for a child than for an adult. After that, the second language is truly a challenge, and may never be spoken as fluently as the mother tongue.

A child of Chinese parents will jabber in the rhythm and intonations of the Chinese language.  A child of Spanish speaking parents will jabber in the rhythm and intonations of the Spanish language. A child who is isolated from language will not learn to speak.  Spoken language is a response to the environment, an effort to get basic needs met, and a way to communicate with others in the group.

A bilingual child will probably be silent for a longer period of time than a monolingual child, however, when that child begins to speak, he or she will most likely speak fluently in either language.  The mother tongue is a huge influence on how we hear language and thus attempt to speak.

French has no /th/ sound, so many native French speakers use the /z/ or /d/ sound to attempt to make the /th/ sound.  English dipthongs such as How (short a---long u) or Joy (long o---long e) are somewhat challenging for speakers of other languages because in most languages, vowels are what they are and are not pronounced in blended combinations.

The best way to acquire English sounds is to listen to speakers of the language and attempt to copy the intonation and sounds of the language.