Learn Finnish: Tää sää on ni hirveä, et mä meen takas sisään. - This weather is so terrible that I am going back inside.

It is heavily shortened because this sentence uses casual spoken Finnish. The word niin (so) has been shortened to just ni, and että (that) is shortened to et.

There are a lot of other shortened words here. What is "meen takas"?

Meen is the spoken form of menen (I go, from the verb mennä). Takas is short for takaisin (back). You will also notice tää instead of tämä (this) and mä instead of minä (I).

Why do we use sisään (into) instead of sisällä (inside)?

Because Finnish location words must match the direction of the action. Since the verb mennä (to go) involves movement toward a destination, you use the "into" form (sisään). If you were already staying inside, you would use sisällä.

Is the comma before et mandatory?

Yes. Finnish punctuation rules are strict about this. You must always put a comma before että (or its spoken form et) when it introduces a subordinate clause, even though we often leave out the comma before "that" in English.