Learn Finnish: Hän on iloinen.

In Finnish, the pronoun hän is completely gender-neutral—it means both 'he' and 'she'! Because this single sentence has no other context, it could just as easily be translated as 'He is happy.'

Why is the verb on used here instead of olen?

On is the form of the verb 'to be' that means 'is'. It always pairs with hän (he/she). In this lesson, you are also learning olen (am) which only goes with minä (I), and olet (are) which goes with sinä (you).

Could I just say On iloinen and leave out hän, like I can leave out minä?

No, you usually should not drop hän. While you can safely drop the pronouns minä (I) and sinä (you) because their verb endings make it obvious who is acting, the word on doesn't give enough information on its own. You need hän to clarify who you are talking about.

I have heard Finns say Se on iloinen when talking about a person. Is that wrong?

Grammatically, se means 'it' and hän means 'he/she'. In standard written Finnish, you must use hän for people. However, in casual spoken Finnish, it is extremely common to use se for people! Both are good to know.