Verbseoul.
하다 verb (adj)
진지하다jinjihadasincere, earnest

To say sincere, earnest politely in the present tense, use 진지해요 (cheen-chee-hae-yoh). Below is the full conjugation of 진지하다 across the three tenses and three everyday speech levels.

Conjugation table

TenseCasual (반말)Polite (요)Formal (합니다)
Present
진지해
cheen-chee-hae
진지해요
cheen-chee-hae-yoh
진지합니다
cheen-chee-hahm-nee-dah
Past
진지했어
cheen-chee-hae-ssuh
진지했어요
cheen-chee-hae-ssuh-yoh
진지했습니다
cheen-chee-hae-sseum-nee-dah
Future
진지할 거야
cheen-chee-hahl- -guh-yah
진지할 거예요
cheen-chee-hahl- -guh-yae-yoh
진지할 겁니다
cheen-chee-hahl- -guhm-nee-dah

Other useful forms

Command Polite (세요)
진지하세요
cheen-chee-hah-sae-yoh
Suggestion / Let's Polite (요)
진지해요
cheen-chee-hae-yoh
Question Polite (요)
진지해요?
cheen-chee-hae-yoh-?
Connective “and” — (고)
진지하고
cheen-chee-hah-goh
Conditional “if” — (면)
진지하면
cheen-chee-hah-myuhn
Noun form — (기)
진지함
cheen-chee-hahm