Learn Finnish: Ne tapailee nyt. - They are dating now.

In colloquial Finnish (puhekieli), the pronoun for "they" is ne (instead of the standard he). A major rule of spoken Finnish is that ne always takes the 3rd-person singular verb form. That is why we use ne tapailee rather than the plural ne tapailevat.

What is the difference between tapaa and tapailee?

Tapaa comes from the verb tavata (to meet) and refers to a single meeting. Tapailee comes from tapailla, which means "to meet repeatedly" or "to date casually." The -ile- in the middle makes it a frequentative verb, showing that the action is ongoing or happens often.

How would you write this exact sentence in formal Finnish (kirjakieli)?

You would write He tapailevat nyt. Formal written Finnish uses he for "they" when referring to people, and requires the matching plural verb ending -vat.

Why doesn't the verb tapailee need an object here?

Just like the English verb "to date," tapailla can be used as a mutual action without an object when it means the subjects are dating each other. If you wanted to say one person is dating a specific person, you would add that person in the partitive case, for example: Se tapailee Mattia (He/She is dating Matti).