Like many other languages, English offers certain ways to make expressions shorter and hence easier to say and write. Contractions such as
don’t (= do not)
won’t (= will not)
isn’t (= is not)
are widely used especially in speech, although they are sometimes still discouraged in formal academic writing (at least in technical fields).
Importantly, contractions also occur in the pronunciation of English words, a fact that is often missed by non-native speakers. For example, the word
is correctly pronounced as [ˈɪntrəst] or as [ˈɪntrest], but *not* as [ˈɪnterest] (the same applies to interesting). This contraction is not optional!
Another example is the suffix -al in words such as
which end in [kl]. The same principle applies to adverbs such as
all ending in [kli]. If you pay attention, you will notice that many non-native speakers pronounce these words with the ending [eli], as in belly.
The phenomenon of contractions is very common in English, but not universal (for example, internationally ends in [əli]; whether or not a contraction occurs depends on the rest of the word). Hence, you will have to listen closely or look up words in a dictionary to avoid mistakes. However, implementing the correct contractions will allow you to significantly improve your pronunciation.