Learn Finnish: Nämä ovat liput, ja minä ostan myös kartan. - These are the tickets, and I will also buy a map.

Kartan is the object of the verb ostan (I buy). As you are learning in this lesson, when you do a complete action to one whole thing in a positive sentence, the object gets an -n ending. This is called the genitive-accusative object.

Why does kartta lose a t when it becomes kartan?

This happens because of consonant gradation, a common spelling change you have seen in earlier lessons. When you add an ending that closes the final syllable, like the -n, the strong double tt in the middle of the word softens into a single weak t.

If kartan gets an -n ending, why doesn't liput get one?

There are two reasons! First, liput is connected to the verb olla (to be: ovat). The verb 'to be' just describes what something is, so it doesn't take an action object with an -n. Second, liput is plural (tickets), and plural words never take this -n object ending.

The English translation says I will also buy, but ostan is present tense. How do we know it happens in the future?

Finnish doesn't have a separate future tense, so the present tense handles both what is happening now and what will happen later. The -n ending on kartan actually helps signal this: it shows you will completely finish the action of buying the map, which strongly hints that it is a planned future action.