The Reflexive Pronoun: Себя
The reflexive pronoun себя means "oneself" (myself, yourself, himself, ourselves, themselves). Just like свой, it always points back to the subject of the sentence.
Because it always receives the action, себя has no Nominative case (it can never be the subject of a sentence). It never changes based on person, gender, or number, but it does change case depending on its role in the sentence.
Себя in the Accusative Case
When the subject is the direct object of its own action, use the Accusative form: себя.
In Russian, several common verbs require себя where English does not use a reflexive pronoun. For example, you must say "to feel oneself" (чувствовать себя) and "to behave oneself" (вести себя).
You also use себя after prepositions that take the Accusative case, such as looking at yourself (на себя), or as the direct object of verbs like taking care of yourself (беречь себя).
Direct Actions on Oneself
For verbs like loving, blaming (винить), or pitying (жалеть), себя works exactly like a regular direct object pointing back to the subject.
Себе in the Dative Case
When you do something to or for yourself, the pronoun changes to the Dative case: себе. You will use this when buying something for yourself or leaving something for yourself.
The Dative form себе is also heavily used in common expressions. For example, позволить себе translates to "to afford" (literally: to allow to oneself), and представлять себе means "to imagine" or "to picture" (literally: to present to oneself).