Learn Italian: Devi essere sincero con me. - You must be sincere with me.

In Italian, when you use a modal verb like dovere (must/have to), the main action verb that follows it always stays in the infinitive (dictionary) form. Here, devi is the conjugated modal verb, so essere remains in the infinitive.

Does sincero change if I am speaking to a woman?

Yes. Sincero describes the person you are talking to (the hidden subject 'you'). If you are speaking to a female, the adjective must change to match her gender: Devi essere sincera con me.

Why is it con me instead of con mi?

You must use the stressed pronoun me because it comes right after a preposition (con). Unstressed object pronouns like mi are placed right before the verb or attached to it, but they are never used after prepositions.

Could I use hai bisogno di instead of devi here?

While both can sometimes translate to 'you need to', dovere (used here as devi) expresses rules, obligations, or strong demands. Avere bisogno di is used for a personal physical or emotional need. Since the speaker is demanding honesty as a requirement, devi is the correct and natural choice.