-nout

1. Meaning and Nuance
Beyond just meaning "once," the semelfactive -nout often implies a shorter duration, lesser intensity, or suddenness compared to the base verb (e.g., mazat 'to smear' → máznout 'to smear lightly/quickly'). It is highly productive in modern Czech and is actively used to adapt English loanwords into perfective verbs (e.g., kliknout 'to click', dropnout 'to drop').

2. Morphological Changes
Adding -nout can trigger root vowel shortening (e.g., smýkat → smyknout) or lengthening (e.g., mazat → máznout). Additionally, it frequently causes vowel elision (e.g., polykat → polknout) or the dropping of a syllable/consonant (e.g., hýbat → hnout).

3. Present/Future Tense Formation
These verbs belong to Class II. To conjugate them, drop the -out and add the endings -u, -eš, -e, -eme, -ete, -ou (e.g., kopnout → kopnu, kopneš, kopne...). Since semelfactives are perfective, these forms express future meaning.

4. Past Tense Formation and Register
Forming the past tense (l-participle) depends on the verb's root:

• Vowel roots: Always retain the -nu- syllable (e.g., minout → minul).
• Zero-vowel roots: If a consonant root lacks a full vowel, it must retain the -nu- in the past tense (e.g., hnout → hnul, lnout → lnul) because dropping it would make the word unpronounceable.
• Consonant roots: Traditionally drop the -nu- syllable (e.g., kopnout → kopl, stisknout → stiskl). In the masculine singular, the longer form retaining -nu- (kopnul) is frequently used in speech to avoid a difficult final consonant cluster, but the short form (kopl) remains the primary standard written form. For feminine, neuter, and plural forms, the short forms (kopla, koplo, kopli) are the standard correct forms, while the longer forms (kopnula, kopnulo, kopnuli) are colloquial and considered non-standard.

5. Imperative Formation
When forming the imperative, consonant roots typically take the ending -ni (e.g., kopnout → kopni), while vowel roots take -ň (e.g., minout → miň).

6. Important Distinction
Do not assume all verbs ending in -nout are perfective semelfactives! Many verbs ending in -nout are imperfective, typically denoting an ongoing process or a gradual change of state (e.g., stárnout 'to age', chladnout 'to cool').