Learn Finnish: On turvallista linnoittautua kellariin. - It is safe to barricade oneself in the basement.

It comes from the verb linnoittaa (to fortify). By replacing the ending with the reflexive suffix -utua, the action reflects back on the subject. Instead of fortifying a building, you are fortifying yourself—meaning to barricade oneself.

Why is the illative case used for kellariin when the English translation says "in the basement"?

In Finnish, verbs that involve settling, locking, or hiding yourself somewhere take a destination case (like the illative -iin), because you are moving yourself into that state or place. You will see the same logic with similar reflexive verbs like piiloutua (to hide oneself).

Why is turvallista in the partitive case rather than the nominative turvallinen?

In sentences following the "It is [adjective] to do [verb]" structure, Finnish treats the action as an abstract, uncountable concept. Because you are describing the abstract action of barricading, the adjective evaluating it must be in the partitive case.

How does consonant gradation affect the -utua suffix when conjugating linnoittautua?

The suffix itself undergoes consonant gradation. While the strong tt in the root word stays the same, the t in the -utua suffix weakens to d when the syllable is closed by a personal ending. For example, "I barricade myself" becomes linnoittaudun.