The biggest difficulty of Vietnamese learners of English in learning English is expressing stress by tone. In Vietnamese, each syllable is pronounced with its characteristic tonal pattern. Without its tone, the syllable is not recognizable.
There are six different tones in Vietnamese. Five of them are indicated in the Romanized script by five diacritical marks. The absence of a diacritic indicates the sixth tone. Each syllable contains one of the five diacritic (or no diacritic) and it is pronounced according to the type of diacritic used.
In compound words, formed by two or more syllabic units, each syllable retains its own distinctive tone. Because each syllable has its own tone, the Vietnamese learners of English tend to give to every English syllable its own tone (usually full stress over each syllable of the word). Their ears are trained to look for tones and distinguish the word according to its tone. This is the main reason for their terrible English listening skills (they can’t find the tones they are looking for).
While it is relatively easy for any Vietnamese learner of English to say black bird, they find it almost impossible to say blackbird (with the stress only on the first syllable).
Vietnamese learners of English find English intonation patterns very difficult to hear and almost impossible to reproduce.
Example 1 (high level tone)
Speaker A: “I beg you pardon.”
Speaker B: “Yes?”
Example 2: (mid falling tone)
Speaker A: “Are you going to?”
Speaker B: “Yes.”
Because none of these is similar to a Vietnamese tone, it is not immediately recognizable. That’s why, in many situations, even a simple question can be very confusing for the Vietnamese learners. The confusion and difficulty becomes even greater when the meaningful pattern of tones and stress extends to group of words that have different meaning in phrases comprising the same English words.