Verbseoul.
하다 verb (adj)
단단하다dandanhadato be hard, solid, firm (resistant to pressure)

To say to be hard, solid, firm (resistant to pressure) politely in the present tense, use 단단해요 (dahn-dahn-hae-yoh). Below is the full conjugation of 단단하다 across the three tenses and three everyday speech levels.

Conjugation table

TenseCasual (반말)Polite (요)Formal (합니다)
Present
단단해
dahn-dahn-hae
단단해요
dahn-dahn-hae-yoh
단단합니다
dahn-dahn-hahm-nee-dah
Past
단단했어
dahn-dahn-hae-ssuh
단단했어요
dahn-dahn-hae-ssuh-yoh
단단했습니다
dahn-dahn-hae-sseum-nee-dah
Future
단단할 거야
dahn-dahn-hahl- -guh-yah
단단할 거예요
dahn-dahn-hahl- -guh-yae-yoh
단단할 겁니다
dahn-dahn-hahl- -guhm-nee-dah

Other useful forms

Command Polite (세요)
단단하세요
dahn-dahn-hah-sae-yoh
Suggestion / Let's Polite (요)
단단해요
dahn-dahn-hae-yoh
Question Polite (요)
단단해요?
dahn-dahn-hae-yoh-?
Connective “and” — (고)
단단하고
dahn-dahn-hah-goh
Conditional “if” — (면)
단단하면
dahn-dahn-hah-myuhn
Noun form — (기)
단단함
dahn-dahn-hahm