-ся/-сь

The reflexive suffix has two forms used to maintain the melody (euphony) of the language:

• -ся: Standardly required in formal writing after consonants and the soft sign (ь). For example: вмивався (he washed himself), миються (they wash themselves). It is also used after vowels if the following word begins with a consonant. At the end of a sentence, -ся is the standard default in formal writing, though -сь is also possible.
• -сь: Used after vowels if the following word begins with a vowel. For example: Я миюсь уранці (I wash myself in the morning). In colloquial speech and poetry, it is frequently used after consonants as well (e.g., вмивавсь).
• Infinitives: Standard dictionary infinitives always use -ся (e.g., митися), though -тись is common in spoken language.
• Gerunds (Adverbial Participles): These almost exclusively take -сь in modern standard Ukrainian (e.g., сміючись), regardless of the following sound.

Important Syntactic Rule: Adding the -ся/-сь suffix generally makes a verb intransitive, meaning it usually cannot take a direct object in the accusative case without a preposition. However, there are highly common exceptions like дивитися (телевізор) and слухатися (маму) which can take direct objects.

Functions:
1. Reflexive: The subject does the action to themselves (e.g., dressing oneself).
2. Indirect Reflexive: The subject performs an action for their own benefit or interest (e.g., будуватися - to build for oneself, збиратися - to pack one's things).
3. Reciprocal: Two or more people do the action to each other (e.g., meeting each other).
4. Passive: The subject receives the action (e.g., the house is being built). Crucial Stylistic Note: In modern standard Ukrainian, using -ся for passive constructions with an active agent in the instrumental case (e.g., будинок будується робітниками) is considered a severe stylistic error. Standard Ukrainian strongly prefers the active voice (робітники будують будинок) or impersonal -но/-то forms.
5. Intransitive/State: Some verbs only exist with this suffix, known as Reflexiva Tantum (e.g., to laugh, to fear).
6. Active-Non-Object: Describes a permanent quality or habit (e.g., 'the dog bites').
7. Impersonal: Describes a state or desire without a clear subject, often used with the dative case (e.g., 'I feel like/I can't...').

Pronunciation & Spelling Note: In the 3rd person (ending in -ться), the combination is pronounced as a long, soft 'ts' sound [t͡sʲːa]. Note that the letter т is not pronounced as a 't' at all; it is fully absorbed into the long 'ts' sound. In the 2nd person (ending in -шся), it is pronounced as a long, soft 's' sound [sʲːa]. Crucially, the ending -ться is always written with a soft sign, whereas -шся is NEVER written with a soft sign.