20–90 and the Special ยี่สิบ

Thai forms the multiples 30–90 as a basic digit plus สิบ: สามสิบ “30,” สี่สิบ “40,” and so on. Twenty is the one special tens form: ยี่สิบ, never สองสิบ in the standard cardinal system. ยี่ is not a free alternative for “two”; it appears here as the tens multiplier.

To add a ones digit, put it after the tens expression. Compounds ending in one use เอ็ด: ยี่สิบเอ็ด “21,” สามสิบเอ็ด “31.” Other final digits keep their ordinary forms: ยี่สิบสอง “22,” สามสิบห้า “35.” The entire cardinal then comes before the classifier.

The multiples of ten

Thai writes each cardinal as a continuous Thai word. RTGS can separate the components for reading, but it does not show tone or vowel length.

No word for zero is added to an exact multiple of ten: 40 is สี่สิบ, not สี่สิบศูนย์. The structure itself identifies the empty ones place.

Minimal contrast: two versus twenty

สอง is the independent cardinal two and the ones digit two. ยี่ appears only as part of the tens formation ยี่สิบ in this range.

The noun-number-classifier frame stays the same. Only the cardinal changes from สอง to ยี่สิบ; neither the noun nor the verb receives plural agreement.

Building numbers between the tens

Place the ones digit after the tens expression. The final-one form is เอ็ด, not หนึ่ง. This page applies the pattern only within 20–99; larger place values have their own formation rules.

เอ็ด is a positional form within a compound, not a classifier and not a formal version of หนึ่ง. See 11–19 and the special เอ็ด for the first occurrence, สิบเอ็ด.

Complete noun phrases with tens

The complete cardinal occupies the number slot in noun + number + classifier. The classifier follows the entire multi-part number.

The words จะ and แล้ว locate events in time; the verbs do not conjugate for number or tense. English plural books/houses/questions corresponds to no plural suffix on the Thai nouns. Numerals and classifiers express the count.

Demonstratives and exact number

When an exact counted set is also identified, the demonstrative follows the classifier: เสื้อยี่สิบตัวนี้ “these twenty shirts.” It does not precede the noun and does not interrupt the cardinal.

Without the numeral-classifier unit, a bare noun can leave quantity open. Without the demonstrative, เอกสารสามสิบชุด is still an exact count but not explicitly anchored as “these/those.” Thai packages number and deixis in successive slots rather than using articles or inflection.

Spoken and written conventions

The same forms are used in (informal) conversation, ordinary polite speech, and (formal) reading of cardinal quantities. ASCII digits are common in prices, schedules, forms, and tables; when read as quantities, they take the cardinal forms taught here.

A digit string is different. A code written with 2 followed by 0 may be read สอง ศูนย์, digit by digit, rather than ยี่สิบ. Phone, address, account, and room-number conventions belong on their dedicated page. Do not infer number formation from how a code is recited.

Ordinary-polite dialogue

The classifier ชุด remains in both counts, while the known noun เอกสาร and the agents are omitted. Thai does not require English-style we or passive tense morphology.

Common Mistakes

1. Forming twenty as “two tens” mechanically

2. Using ยี่ as standalone two

3. Ending 21 with หนึ่ง

4. Putting the classifier inside the cardinal

Key Takeaways

Twenty is the special ยี่สิบ; 30–90 use ordinary digit + สิบ. Add the ones digit after the tens expression, replacing final หนึ่ง with เอ็ด in compounds such as 21 and 31. Keep the complete number before the lexical classifier, then place any demonstrative after the classifier. Codes read digit by digit follow a different convention.