Short adjectives as predicates

Short adjectives are formed by removing the endings from long adjectives and adding specific gender/number markers:

• Masculine: Zero ending. If the stem ends in two consonants, a 'fleeting vowel' (о, е, or ё) is often inserted between the final two consonants of the stem (e.g., умный → умён, смешной → смешон, светлый → светел, острый → остёр or остр, хитрый → хитёр). However, some adjectives ending in a consonant followed by -р or -л omit the fleeting vowel (e.g., добрый → добр, быстрый → быстр, круглый → кругл, подлый → подл). Note: Masculine short adjectives ending in hushing consonants (ж, ш, ч, щ) never take a soft sign (ь) (e.g., хороший → хорош, горячий → горяч, свежий → свеж).
• Feminine: Ending -а. Note that the stress often shifts to this final syllable (e.g., умна, права). Masculine, neuter, and some plural forms typically retain the stress on the stem (e.g., право, правы), while other plural forms frequently shift stress to the ending or have variant stress (e.g., умны).
• Neuter: Ending -о or -е (e.g., красиво, искренне). -е is used when the adjective's stem ends in a soft consonant (usually adjectives ending in -ний, like искренний → искренне, синий → сине). Note that the neuter short form looks identical to an adverb, but syntactically, adjectives modify nouns/pronouns while adverbs modify verbs.
• Plural: Ending -ы or -и (e.g., умны, велики). Note that -и is used after velar (г, к, х) and hushing (ж, ч, ш, щ) consonants due to the 7-letter spelling rule.

Usage Rules:
1. Predicate Position: In modern Russian, they almost exclusively function as the predicate. They are rarely used as modifiers before a noun, except in a few fixed expressions and folklore idioms (e.g., на босу ногу, средь бела дня).
2. State vs. Trait: In everyday speech, short forms often describe temporary states or specific situations, while long forms describe permanent traits. However, in formal, scientific, or literary contexts, short forms are frequently used to describe permanent traits (e.g., Эта теорема сложна).
3. Excessive Quality: Short forms are frequently used to indicate that a quality is 'too much' for a specific purpose (e.g., эти туфли малы - these shoes are [too] small).
4. Modal Meanings: The adjective рад (glad) exists exclusively in the short form. Adjectives like должен (must), готов (ready), согласен (agree), намерен (intend), прав (right), обязан (obliged), and виноват (guilty) function almost exclusively in the short form to express states or modality. The adjectives 'большой' and 'маленький' have irregular short forms: велик and мал.
5. Dependent Words: If an adjective has dependent words, the short form is strongly preferred or required. Short adjectives often require specific cases or prepositions (government) that long forms do not (e.g., Стакан полон воды, болен гриппом, похож на отца, согласен с другом, готов к экзамену).
6. Meaning Shifts: Some adjectives change their dictionary meaning when made short (e.g., правый means 'right side', but прав means 'correct').
7. Qualitative Only: While only adjectives describing qualities can have short forms, relative (e.g., русский, деревянный) and possessive (e.g., мамин, лисий) adjectives never do. Furthermore, not all qualitative adjectives have them. For example, colors derived from nouns (like оранжевый), adjectives with certain suffixes (like -ск-, -ов-), adjectives denoting animal suits (e.g., вороной, гнедой), and adjectives with suffixes of subjective evaluation (e.g., злющий, предобрый) lack short forms.
8. Case Restriction: Short adjectives do not decline by case. When used with the past or future tense of 'быть', they must remain in the Nominative case (e.g., Он был болен), whereas long adjectives in the predicate often take the Instrumental case (e.g., Он был больным).
9. Stylistic Register: Short adjectives often carry a more formal, bookish, or categorical tone compared to long adjectives in the predicate, which are more neutral or colloquial (e.g., Она очень добра vs. Она очень добрая).
10. Spelling of -Н- vs. -НН-: In feminine, neuter, and plural forms, short adjectives retain the double -нн- of their full form (e.g., девочка умна и воспитанна), which distinguishes them from short passive participles that always take a single -н- (e.g., девочка воспитана бабушкой). Note that in the masculine short form, adjectives ending in -енный usually drop one 'н' to end in -ен (e.g., уверенный → уверен, естественный → естествен).