Polite and casual are social choices
Korean speech level communicates more than grammar. Age, closeness, role, and the situation all matter. That is why a perfectly formed casual sentence can still be socially wrong for the moment.
Polite → casual
Polite → casual
Do not just delete 요 every time
Some common forms reshape when they become casual. For example, 이에요/예요 usually becomes 이야/야, and polite requests or honorific expressions may need a different sentence entirely rather than a mechanical deletion.
When in doubt, stay with -요
Polite -요 style is flexible and friendly enough for most everyday learner situations. Moving to 반말 later is much easier than repairing a first impression made with speech that was too casual.
How people agree to use 반말
A relationship may shift to casual speech after people become closer, but age alone does not give a learner permission to make the switch. Listen to how the other person speaks and, when needed, ask directly. If the answer or situation is unclear, continuing with -요 is completely normal.
A polite way to suggest switching speech levels.
Permission still depends on the relationship and context.
Requests need more than removing 요
Turning a polite request into a casual one often changes the ending or the whole expression. ‘앉으세요’ is honorific and polite; with a close friend, ‘앉아’ is the natural casual form. Mechanical deletion would produce the wrong result.