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HSK 4 Fixed Four-Character Phrases: The 一A一B Pattern
The four-character pattern 一A一B places 一 in the first and third positions with related or contrasting characters in the other two. Learn familiar expressions as whole units: their meanings can include concentration, every detail, or contrasting and alternating aspects.
他对这份工作一心一意。
一心一意 describes his wholehearted devotion to the work. Learn its established meaning as a whole expression.
Read the full explanation and examples
What Is the 一A一B Pattern?
In the HSK 4 grammar outline, the pattern 一A一B is categorized under fixed phrases (固定短语) and four-character expressions (四字格). Note that this level designation reflects the supplied syllabus outline rather than a guarantee of what appears on any specific test administration. Structurally, this template takes the numeral 一 (one) and repeats it in the first and third positions, while placing two distinct single-character morphemes in the second (A) and fourth (B) positions.
Compare 一A一B with repetition such as 一步一步, “step by step”. In 一A一B the two positions contain different characters. Many familiar expressions, such as 一心一意, are established idioms; other combinations, such as 一高一低, can describe a particular scene. Learn the meaning and sentence use of the whole phrase.
Common Meaning Groups
These groups help you remember examples; they are not a complete classification or a rule for calculating every expression from its characters.
- Sameness and concentration (A and B are near-synonyms): Here, 一 means 'the same' or 'unified'. In phrases like 一心一意 (one heart and one mind) or 一模一样 (the exact same mold and appearance), pairing two similar nouns emphasizes total focus or complete identity.
- Every single item or totality (A and B are representative nouns or actions): Here, 一 means 'each' or 'every smallest unit'. In phrases like 一言一行 (every word and action), 一举一动 (every movement), or 一草一木 (every blade of grass and tree), the two concrete items stand for the entire category, meaning even the smallest detail is included.
- Alternation or contrast (A and B are antonyms or paired counterparts): Here, the two 一 characters point to two different sides or consecutive states, meaning 'one does A, the other does B' or 'first A, then B'. Common examples include 一高一低 (one high, one low), 一前一后 (one in front, one behind), and 一唱一和 (one sings, the other chimes in).
他对这份工作一心一意。
一心一意 describes his wholehearted devotion to the work. Learn its established meaning as a whole expression.
父母的一言一行都会影响孩子的成长。
一言一行 (every word and deed) acts as the subject noun phrase modified by 父母的.
这两座山一高一低，远远看去很有特色。
一高一低 pairs two contrasting single-character adjectives to describe two items side by side.
How to Use 一A一B in a Sentence
Because 一A一B covers different types of morphemes, you cannot use every 一A一B phrase in the exact same grammatical slot. Its role in a sentence depends on whether the whole expression functions like a descriptive state, an adverbial manner, or a noun phrase.
一心一意 and 一五一十 commonly modify a following verb, sometimes with 地: 一心一意地工作 or 一五一十地说清楚. 一言一行 and 一草一木 commonly form noun phrases used as subjects or objects. 一唱一和, with 和 pronounced hè, describes people echoing or backing each other up; learn its usage separately rather than placing every phrase before a verb in the same way.
张老师一心一意地帮助每一个学生。
Followed by 地, 一心一意 serves as an adverbial modifier before the verb 帮助.
请不要随便破坏森林里的一草一木。
一草一木 serves as the object of the verb 破坏, representing all plants and small parts of the forest.
Fixed Expressions and Literal Paraphrases
It is helpful to contrast 一A一B with two closely related patterns. First, compare it with 一A一A (such as 一步一步 or 一个一个). In 一A一A, the same measure word or noun is repeated to show steady progression one unit at a time. When you switch to a four-character phrase with two different morphemes like 一步一景 (every step brings a new view) or 一朝一夕 (a single morning or evening, meaning a short time), the expression becomes more literary and compact.
For established idioms, learn the conventional characters and order. To express care for all the vegetation, 一草一木 is a familiar choice. You can also state the meaning explicitly with 每一株草、每一棵树. A newly coined combination may be understandable or poetic, but that does not make it the established idiom learners need in neutral prose.
请爱护校园里的一草一木。
The established phrase 一草一木 emphasizes every part of the vegetation. Compare the explicit wording with 每 in the next example.
请爱护校园里的每一株草、每一棵树。
This is a correct plain-language paraphrase of the preceding example, not an error correction. 株 and 棵 count individual plants and trees here.
Frequently asked questions
How does the tone of 一 change in 一A一B phrases?
Reference pinyin here keeps the base tone yī. In ordinary connected speech, 一 usually becomes yì before tones 1–3 and yí before tone 4. Thus 一心一意 commonly sounds like yì xīn yí yì, and 一言一行 like yì yán yì xíng. The written base tone and the spoken change describe different layers.
Can I switch the order of A and B in a fixed 一A一B phrase?
For a familiar idiom, learn its conventional order, such as 一心一意 or 一草一木. Reversing characters may change the expression or its tone. In literal descriptions such as 一大一小 and 一小一大, the order can follow which object you introduce first.
Can I put 很 (hěn) before an 一A一B phrase?
Check the whole expression. 很 cannot simply modify a noun phrase such as 一言一行, and 一模一样 already means “exactly the same”. To say someone is very focused, 非常专心 is a clear choice. The numeral 一 alone does not determine whether a degree modifier is possible.
