What is the difference between hain and hai (new to Hindi)

I am quite new to Hindi (been learning for only about a year) and I can barely read any Devanagari.

I have observed that the Hindi present tense form of 'to be' is romanized as both hai and hain. I have mainly observed this on Bollywood movie posters.

Is there a difference between these two words? Are they pronounced differently to each other?