How do Korean speech styles change statements into questions or commands?

In the Korean language, the function of a predicate—whether it serves as a statement, question, or command—is determined by specific grammatical endings and, in some cases, intonation.1

Formal Style

In the formal speech style, the predicate ending explicitly defines the sentence type:1
* Statement: Marked with -(sŭ)mnida.1
* Question: Marked with -(sŭ)mnikka.1
* Command: Marked with -(sŭp)sio.1
* Suggestion: Marked with -(sŭ)psida.1

Plain/Impersonal Style

The plain style utilizes different suffixes to distinguish these functions:1
* Statement: Uses -da or the processive -(nŭ)nda.1
* Question: Uses -na, -ni, or -(nŭ)n-ya/-ka.1
* Command: Uses -(ŭ)ra or the infinitive form plus -ra.1
* Suggestion: Uses -ja/-cha.1

Informal Style

In informal speech, the distinction between statements and questions often relies on intonation rather than distinct endings. For instance, a processive predicate like anja can function as a statement, question, command, or suggestion depending on how it is spoken. Additionally, the particle -yo is frequently added to the infinitive form to convey friendly politeness. Note that while processive predicates (verbs) can be used for commands and suggestions, descriptive predicates (adjectives) are generally limited to making statements or questions.1