cực kỳ / vô cùng

1. What they attach to:
They modify adjectives (khó, đẹp) and psychological verbs (thích, biết ơn, cảm động).

2. Position in the sentence:
While often introduced to beginners as going before the adjective or psychological verb (e.g., cực kỳ ngon, vô cùng thích), native speakers frequently and naturally place them after the adjective or psychological verb in both casual speech and literature (e.g., ngon cực kỳ!, thích vô cùng!). Placing them after the word typically creates an exclamatory sentence expressing strong emotion, surprise, or personal realization. When a psychological verb has a direct object, the adverb goes before the verb (Tôi cực kỳ thích cuốn sách này) or after the object (Tôi thích cuốn sách này cực kỳ), but not between the verb and the object (Tôi thích cực kỳ cuốn sách này is incorrect).

3. Nuance differences and special usages:

• Cực kỳ (also spelled cực kì): Great for everyday, measurable extremes (e.g., cực kỳ khó - extremely difficult). Both spellings are officially accepted and used interchangeably. It is heavily used in casual spoken Vietnamese. In casual speech, it is often shortened to just cực, which is frequently used both before (e.g., cực ngon, cực thích) and after the adjective (e.g., ngon cực!). Speakers also frequently add the particle luôn for extra emphasis (e.g., ngon cực kỳ luôn!).
• Vô cùng: Better for boundless, abstract, or profound states (e.g., vô cùng thương tiếc - deeply mourn). This makes sense given its Sino-Vietnamese roots (vô = without, cùng = end/limit). It leans slightly more formal and literary. Additionally, vô cùng can function as a standalone adjective meaning "infinite" or "endless" (e.g., vũ trụ vô cùng - the infinite universe).

4. Grammatical restrictions:
Because they represent the absolute limit, they cannot be used with attenuating reduplication (adjectives that mean "slightly," like đo đỏ). You also cannot stack them with other degree adverbs like rất / lắm / quá. Furthermore, they cannot modify regular action verbs directly; they must modify an adverb of manner instead (e.g., chạy cực kỳ nhanh - run extremely fast, not just chạy cực kỳ). Finally, both cực kỳ and vô cùng sound highly unnatural when directly negated (e.g., không cực kỳ ngon or không vô cùng đẹp). Native speakers default to không [adjective] lắm or không [adjective] cho lắm instead.