Learn Finnish: Trendi pysyy suosittuna. - The trend stays popular.

This is the essive case, which is required by the verb pysyä (to stay / remain). When you talk about staying in a certain state, Finnish treats it as 'staying as something'. Therefore, suosittu (popular) becomes suosittuna.

How would this sentence look if the trend became popular instead of staying popular?

To express a change into a new state, you use verbs like muuttua (to change or become) along with the translative case (ending in -ksi). So, 'The trend becomes popular' would be Trendi muuttuu suosituksi.

Suosittu looks like a verb participle. Does it behave like a normal adjective here?

Exactly! It is originally the passive past participle of the verb suosia (to favor), literally meaning 'favored'. However, it has become a standard adjective for 'popular'. Because it acts like a normal adjective, it takes case endings like the essive -na.

Why doesn't the double tt in suosittu drop to a single t before the -na ending?

The essive case is a strong case in Finnish. This means it attaches directly to the strong stem of the word without triggering consonant gradation (weakening). So, the strong tt remains intact in suosittuna.