Oblique personal pronouns

In Russian, personal pronouns must match the case required by the verb or preposition they follow.

1. 1st and 2nd Person (я, ты, мы, вы):
These pronouns change significantly. For example, я becomes меня (Gen/Acc), мне (Dat/Prep), and мной (Inst). Similarly, ты becomes тебя, тебе, тобой. Plural forms also change: мы becomes нас (Gen/Acc/Prep), нам (Dat), and нами (Inst), and вы becomes вас (Gen/Acc/Prep), вам (Dat), and вами (Inst).

2. 3rd Person (он, она, оно, они):
The base oblique forms for these pronouns are: его (Gen/Acc), ему (Dat), им (Inst), (о) нём (Prep) for он/оно; её (Gen/Acc), ей (Dat), ей/ею (Inst), (о) ней (Prep) for она; and их (Gen/Acc), им (Dat), ими (Inst), (о) них (Prep) for они. These pronouns have a special rule: when they follow a simple preposition, they add an initial н-.

• Without preposition: Я вижу его (I see him).
• With preposition: Я иду к нему (I am going to him).
• Plural: их (Gen) becomes у них (they have).

Note: The н- is always present in the prepositional case, mapping to their nominative counterparts: нём (он/оно), ней (она), них (они). It is also added after most derivative prepositions governing the genitive (e.g., после неё). For prepositions like вне, the strict rule forbids the н- prefix (вне его), while внутри allows both forms equally (внутри его / внутри него). For many compound prepositions formed from a noun, the н- prefix is the established norm (e.g., с помощью него is standard, and в отношении его / в отношении него are equally correct). The strict literary rule forbids н- after prepositions governing the dative (e.g., благодаря ему, вопреки ему, навстречу ей, наперекор им, согласно им, подобно им); while forms with н- are frequently heard in everyday colloquial speech, they are considered grammatical errors in standard Russian. In comparative constructions, while traditionally the н- prefix was not added when the pronoun followed a comparative adjective or adverb, modern Russian accepts both forms (e.g., старше его / старше него, лучше её / лучше неё, больше их / больше них). Additionally, if the pronoun is separated from the preposition by another word (like весь or a conjunction), the н- prefix is traditionally dropped or becomes optional (e.g., у всех их / у всех них, or между мной и им). Finally, remember that 3rd-person possessive pronouns (его, её, их) never take the н- prefix, even after prepositions (e.g., в его доме). This н- prefix is often the only way to disambiguate personal and possessive pronouns in a sentence (e.g., Я был у него [I was at his place] vs. Я был у его брата [I was at his brother's place]).

3. Phonological Changes:
Some prepositions add an -о before certain pronoun forms for easier pronunciation. The most common is обо мне (about me), as well as со мной (with me) or ко мне (to me).

4. Instrumental Variants:
Forms like мной and тобой have longer literary variants (мною, тобою), but the short forms are standard in modern speech.