Learn Finnish: Kartan mukaan koulu on täällä. - According to the map, the school is here.

Kartan is the genitive form of kartta. The word mukaan is a postposition, which works like an English preposition but comes after the noun. In Finnish, most postpositions require the noun before them to take the genitive case, which is why we add the -n ending.

Why does kartta lose a letter when it becomes kartan?

This happens because of a rule called consonant gradation. When you add the genitive ending -n to kartta, it closes the final syllable. This causes the strong double consonant tt to weaken into a single t.

Doesn't mukaan usually mean "along" or "with"?

It does! You will often hear it in phrases like tule mukaan, meaning "come along". However, when you use mukaan as a postposition right after a word in the genitive case, it translates to "according to".

Why use täällä for "here" instead of tässä?

Both words mean "here", but they cover different sizes of space. Tässä is for a very specific, exact spot. Täällä refers to a larger or more general area, like "around here". Because a school is a large place, täällä is the most natural choice.