-нный/-енный

The Past Passive Participle (PPP) is a 'verbal adjective' that focuses on the result of a completed action. In Russian, these are primarily formed from transitive perfective verbs.

1. Formation Rules:

• -нный: Used for verbs ending in -ать or -ять. The infinitive vowel is kept. Example: прочитать → прочитанный (read).
• -енный / -ённый: Used for verbs ending in -ить, -еть, -ти, -чь, -сть, or -зть. For -ить and -еть verbs, the infinitive vowel is dropped (e.g., построить → построенный). For verbs ending in -ти, -чь, -сть, or -зть (consonant stems), the suffix is added to the future tense stem (e.g., они украд-ут → украд- + -енный = украденный). Stress shift for -ить verbs: If the stress falls on the ending in the infinitive, it shifts one syllable back in the participle only if the stress also shifts in the future tense conjugation (e.g., купить → ты купишь → купленный). For verbs with fixed end-stress, the stress falls on the suffix, which becomes -ённый (e.g., решить → ты решишь → решённый). If the infinitive already has stem-stress, the stress remains on the stem (e.g., бросить → брошенный, оставить → оставленный). Stress rule for consonant stems: Verbs ending in -ти and -чь with end-stress in the infinitive almost always take the stressed -ённый suffix (e.g., принести → принесённый, испечь → испечённый).
• -тый: A smaller group of verbs (often monosyllabic or ending in -оть, -уть, -ыть, as well as irregular verbs in -нять and -ереть) use this suffix. Examples: открыть → открытый (opened), понять → понятый (understood), запереть → запертый (locked).

2. Consonant Mutations:
Verbs ending in -ить and certain -еть verbs undergo a consonant shift in the stem (Note: the common verb увидеть is an exception and does not mutate: увиденный):

• д → ж or жд (depending on the root's etymology) (обидеть → обиженный, освободить → освобождённый)
• т → ч or щ (depending on the root's etymology) (отметить → отмеченный, осветить → освещённый)
• ст → щ (простить → прощённый)
• с → ш (пригласить → приглашённый)
• з → ж (поразить → поражённый)
• Labials (в, п, б, м, ф) add an л (купить → купленный)

Verbs ending in -чь have stems ending in к or г, which mutate to ч and ж respectively:

• к → ч (испечь → испечённый)
• г → ж (сберечь → сбережённый)

3. Short Forms and Spelling:
The long form (-нный) is used as an attributive adjective. The short form is used as a predicate to express the passive voice.

• Spelling: The double -нн- of the long form always reduces to a single -н- in the short form (e.g., прочитанный → прочитан). After sibilants (ж, ч, ш, щ), the suffix is always spelled with ё under stress and е when unstressed, and never with о.
• Suffixes: The short form suffix is -н for the first group (прочитан), -ен/-ён for the second group (построен), and -т for the third group (открытый → открыт).
• Agreement: Short forms must agree in gender and number with the subject: -н/-ен/-т (masc), -на/-ена/-та (fem), -но/-ено/-то (neut), -ны/-ены/-ты (plural).
• Stress and ё to е shift: For the -ённый group, the masculine short form retains the ё (e.g., решён), but the feminine, neuter, and plural forms shift the stress to the ending and are spelled with an е (решена, решено, решены). Additionally, many short-form participles shift their stress to the final syllable in the feminine form (e.g., взятый → взята, созданный → создана).
• Tense with быть: Short forms pair with the helper verb быть to express past and future passive tenses (e.g., дом был построен - the house was built, дом будет построен - the house will be built). In the present tense, a zero copula is used (e.g., дом построен - the house is built).

4. Expressing the Agent:
When using the passive voice, the person or thing performing the action is expressed using the instrumental case without a preposition. Example: Книга прочитана студентом (The book has been read by the student).

5. Participles vs. Verbal Adjectives:
While PPPs are primarily formed from perfective verbs and take double -нн- (e.g., поджаренный - fried/toasted), imperfective verbs without prefixes or dependent words form verbal adjectives with a single -н- (e.g., жареный - fried). However, if an imperfective verb has a dependent word, it acts as a participle and takes double -нн- (e.g., жаренный в масле - fried in oil). Note that a few common imperfective verbs can also form past passive participles with double -нн-, often functioning as adjectives (e.g., читанный, виденный).