Learn Finnish: Hän ei lähde mielellään tanssimaan, mutta hän lähtee juhlimaan. - He doesn't like going out to dance, but he goes out to party.

The verb lähteä (to leave, to go out) requires the following verb to be in the third infinitive illative form, which ends in -maan or -mään. This form expresses the action you are going out to do.

Where is the verb for 'like' in this sentence?

There isn't one! Instead of using a verb for 'like' (such as tykätä), Finnish often uses the adverb mielellään, which means 'gladly' or 'with pleasure'. So ei lähde mielellään literally translates to 'does not gladly go'.

Why does mielellään end with -ään?

That ending is a possessive suffix matching the 'he/she' subject (hän). The base word is mielellä (with a mind). Because it is about 'his/her' mind, it gets the third-person suffix -än or -ään. If you were talking about yourself, you would say mielelläni.

Why does the verb change from lähde to lähtee in the second half?

Ei lähde is the negative form. In negative sentences, the main verb drops its personal ending. In the second half, the sentence is positive, so lähtee takes the normal third-person ending, which is formed by doubling the final vowel.