Verbseoul.
하다 verb (adj)
진실하다jinsilhadasincere, honest, faithful

To say sincere, honest, faithful politely in the present tense, use 진실해요 (cheen-sheel-hae-yoh). Below is the full conjugation of 진실하다 across the three tenses and three everyday speech levels.

Conjugation table

TenseCasual (반말)Polite (요)Formal (합니다)
Present
진실해
cheen-sheel-hae
진실해요
cheen-sheel-hae-yoh
진실합니다
cheen-sheel-hahm-nee-dah
Past
진실했어
cheen-sheel-hae-ssuh
진실했어요
cheen-sheel-hae-ssuh-yoh
진실했습니다
cheen-sheel-hae-sseum-nee-dah
Future
진실할 거야
cheen-sheel-hahl- -guh-yah
진실할 거예요
cheen-sheel-hahl- -guh-yae-yoh
진실할 겁니다
cheen-sheel-hahl- -guhm-nee-dah

Other useful forms

Command Polite (세요)
진실하세요
cheen-sheel-hah-sae-yoh
Suggestion / Let's Polite (요)
진실해요
cheen-sheel-hae-yoh
Question Polite (요)
진실해요?
cheen-sheel-hae-yoh-?
Connective “and” — (고)
진실하고
cheen-sheel-hah-goh
Conditional “if” — (면)
진실하면
cheen-sheel-hah-myuhn
Noun form — (기)
진실함
cheen-sheel-hahm