Vietnamese is often erroneously considered to be a "monosyllabic" language. Vietnamese words may consist of one or more syllables. There is a tendency for words to have two syllables (disyllabic) with perhaps 80% of the lexicon being disyllabic. Some words have three or four syllables — many polysyllabic words are formed by reduplicative derivation.
Additionally, a Vietnamese word may consist of a single morpheme or more than one morpheme. Polymorphemic words are either compound words or words consisting of stems plus affixes or reduplicants.
Most Vietnamese morphemes consist of only one syllable. Polysyllabic morphemes tend to be borrowings from other languages. Examples follow:
| Vietnamese word | English gloss | Phonological form | Morphological form |
|---|---|---|---|
| cơm | "cooked rice" | monosyllabic | monomorphemic |
| cù lao | "island" | disyllabic | monomorphemic |
| dưa chuột | "cucumber" | disyllabic | bimorphemic |
| vội vội vàng vàng | "hurry-skurry" | polysyllabic | polymorphemic (reduplicative) |
Most words are created by either compounding or reduplicative derivation. Affixation is a relatively minor derivational process.
Older styles of Vietnamese writing wrote polysyllabic words with hyphens separating the syllables, as in cào-cào "grasshopper", sinh-vật-học "biology", or cà-phê "coffee". Spelling reform proposals have suggested writing these words without spaces (for example, the above would be càocào, sinhvậthọc, càphê). However, the prevailing practice (although considered careless to some) is to omit hyphens and write all polysyllabic words with a space between each syllable.
Reduplication, the process of creating a new word by repeating either a whole word or part of a word, is very productive in Vietnamese (as in other Austro-Asiatic languages), although not all reduplicative patterns remain fully productive.
Its effect is to sometimes either increase or decrease the intensity of the adjective, and is often used as a literary device (like alliteration) in poetry and other compositions, as well as in everyday speech.
Examples of reduplication increasing intensity:
Examples of reduplication decreasing intensity:
A type of assimilation known as tonal harmony is involved in Vietnamese reduplication. The six tones are categorized into two registers:
| Register | Tones |
|---|---|
| Upper | ngang, sắc, and hỏi |
| Lower | huyền, nặng, and ngã |
The tones of all reduplicated words are always within the same tonal register (either upper or lower). For example, nhỏ "small" with the hỏi tone when reduplicated appears as nho nhỏ "smallish" with a ngang-toned reduplicant — both syllables are in the upper tonal register.
Vietnamese has several different types of reduplicative patterns including both total, partial, initial, final, rhyming, and alliterative patterns involving only reduplicated material or both reduplicated material and affixation.
Reduplicant position. The resulting reduplicants can be either initial (preceding the base) or final (following the base).
Total reduplication involves copying the entire word base:
Partial reduplication involves copying only certain segments of the word base. Partial reduplication typically involves the affixation of non-reduplicated segments.
Vietnamese has very limited affixation. Only prefixation and suffixation are attested. A few affixes are used along with reduplication. Many affixes come from the Sino-Vietnamese and learned part of the lexicon. Some examples (not an exhaustive list) follow.
| Prefix | Gloss | Examples |
|---|---|---|
| bán- | "half" | bán nguyệt "semicircular, semi-monthly" (bán- + -nguyệt "moon"), bán đảo "peninsula" (bán- + đảo "island") |
| khả- | "ability" | khả kính "respectable" (khả- + kính "to respect"), khả quan "satisfactory" (khả- + quan "to behold") |
| lão- | familiar (added to surnames) | lão Thinh "ol’ Thinh, good old Thinh" (lão- + Thinh surname) |
| phản- | "counter to, against" | phản cách mạng "counter-revolutionary" (phản- + cách mạng "revolution"), phản chiến "anti-war" (phản- + -chiến "to fight") |
| phi- | "not" | phi nghĩa "unethical" (phi- + nghĩa "righteousness"), phi chính phủ "non-governmental" (phi- + chính phủ "government") |
| siêu- | "above, better" | siêu thị "supermarket" (siêu- + thị "market"), siêu đẳng "outstanding" (siêu- + đẳng "level") |
| tăng- | "over, high" | tăng a xit "hyperacidity" (tăng- + a xit "acid"), tăng can xi "hypercalcernia" (tăng- + can xi "calcium") |
| thứ- | ordinal (added to numerals) | thứ mười "tenth" (thứ- + mười "ten"), thứ bốn mươi ba "forty-third" (thứ- + bốn mươi ba "forty-three") |
| Suffix | Gloss | Examples |
|---|---|---|
| -gia | "profession" | chính trị gia "politician" (chính trị "politics" + -gia), khoa học gia "scientist" (khoa học "science" + -gia) |
| -giả | agentive | tác giả "author" (tác "to create" + -giả), học giả "scholar" (học "to learn" + -giả) |
| -hóa | forms causative verb | a xít hóa "to acidify" (a xit "acid" + -hóa), mỹ hóa "to americanize" (Mỹ "USA" + -hóa) |
| -học | "field of study" | ngôn ngữ học "linguistics" (ngôn ngữ "language" + -học), động vật học "zoology" (động vật "animal" + -học) |
| -kế | "measuring device" | nhiệt kế "thermometer" (nhiệt- "warm" + -kế), áp kế "manometer" (áp "get close, approach" + -kế) |
| -khoa | "field of study" | nha khoa "dentistry" (nha- "tooth" + -khoa), dược khoa "pharmacy" (dược- "drug" + -khoa) |
| -sĩ | "expert" | hoạ sĩ "artist" (hoạ "to draw" + -sĩ), văn sĩ "writer" (văn "literature" + -sĩ) |
| -sư | "master" | giáo sư "professor" (giáo "to teach" + -sư), luật sư "lawyer" (luật "law" + -sư) |
| -viên | agentive | quan sát viên "observer" (quan sát "to observe" + -viên), phối trí viên "coordinator" (phối trí "to coordinate" + -viên) |
| tone alternation | ||
|---|---|---|
| đây "here" | đấy "there" | (ngang tone-sắc tone) |
| bây giờ "now" | bấy giờ "then" | (ngang tone-sắc tone) |
| kia "the other" | kìa "yonder" | (ngang tone-huyền tone) |
Vietnamese also has other instances of alternations, such as consonant mutations and vowel ablaut. Different regional varieties of Vietnamese may have different types of alternations.