Learn Finnish: Teidän täytyy seisoa tiiviisti meidän takanamme.

The postposition takana (behind) is linked to the pronoun meidän (our). In standard Finnish, when you use a personal pronoun with a postposition, you must add the matching possessive suffix to the postposition. Since meidän is first-person plural, it takes the -mme suffix, resulting in takanamme.

Do I always have to use both meidän and the -mme suffix together?

In standard written Finnish, you need the -mme suffix, but you can actually drop the pronoun meidän entirely because takanamme means "behind us" all by itself. In casual spoken Finnish, people usually do the opposite: they say meidän takana and drop the suffix altogether.

Why is the sentence starting with the genitive Teidän instead of Te?

This is because of the verb täytyy (must). In Finnish necessity sentences, the person who "must" do something is always put into the genitive case. Because the base word is te (you, plural or formal), it changes to the genitive teidän.

What kind of word is tiiviisti?

It is an adverb meaning "tightly" or "closely". It is formed from the adjective tiivis (tight). In Finnish, you create adverbs by taking the word's stem, which in this case is tiivii-, and adding the ending -sti. This works just like the "-ly" ending in English.