-ový / -ní

1. Choosing the Suffix:
While there are tendencies, the choice is largely lexicalized. -ový is the highly productive, default suffix for modern loanwords, technology (internetový, e-mailový, softwarový), and modern or processed materials (papírový, plastový), though it is also used for some traditional or natural materials (kovový, pískový). It follows the hard adjective declension (the mladý paradigm), changing by gender (papírový, papírová, papírové). However, for many traditional physical materials, the primary suffixes are -ěný and -ný (dřevěný, skleněný, vlněný, železný). -ní is primarily used for time (ranní), space or location (místní, vnitřní, zahraniční, zadní), as well as abstract concepts, institutions, and activities (pracovní, finanční, životní). It follows the soft adjective declension (the jarní paradigm) and is identical across all genders in the nominative (ranní vlak, ranní káva, ranní slunce). You must learn the specific adjective form for each noun. Note: Other relational suffixes exist, such as -ský/-cký (for places and professions).

2. Consonant Mutations and Epenthesis:
Adding -ní often palatalizes the stem's final consonant: rok → roční, dráha → drážní, ucho → ušní. Sometimes, a mobile -e- is inserted along with a consonant mutation: slunce → sluneční (insertion of -e- and c → č). In contrast, the suffix -ový generally does NOT trigger consonant mutations (e.g., park → parkový), which is why it serves as the default for modern loanwords.

3. Doublets with Different Meanings:
Some nouns can take both suffixes but with distinct meanings. For example, lední relates to an ice environment (lední medvěd = polar bear), while ledový means made of ice or cold (ledový čaj = iced tea). Similarly, vodní (aquatic/water-related) vs. vodový (watery/water-based).

4. Word Order:
Relational adjectives form a tight conceptual unit with the noun. They must "hug" the noun directly. If you add a qualitative adjective (like "new" or "big"), it must precede the relational adjective: nový daňový systém (new tax system).

5. Grading:
Because they define a binary category rather than a spectrum, relational adjectives cannot normally be graded (you cannot say "more tax-related"). However, some develop secondary qualitative meanings that can be graded (e.g., a physical material becoming a metaphor: ledový krystal = ice crystal → ledový pohled = icy stare → ledovější pohled = icier stare).

6. Contrast with Possessives:
Do not confuse these with possessive adjectives. Possessives denote ownership by a specific individual (sousedův = the neighbor's), whereas relational adjectives denote a general category (sousední = neighboring/adjacent).

7. Animal Adjectives in -í:
A major category of soft relational adjectives relates to animals. These take the suffix -í (e.g., pes → psí, kočka → kočičí, medvěd → medvědí). Because they decline exactly like -ní adjectives and are incredibly common, it is important to become confident with them alongside other soft adjectives.