Learn Finnish: Leskikin perii kassakaapin. - The widow inherits the safe too.

In this sentence, kassakaapin is the direct object of the verb periä (to inherit). Because the sentence is affirmative and the action applies to the entire safe, the total object takes the genitive ending -n.

Does the form kassakaapin answer the question kenen (whose) here?

Not in this specific sentence! Even though it uses the exact same -n ending as the possessive genitive (which answers kenen), here it functions as the object of the verb. However, if you wanted to ask "Whose safe does the widow inherit?", you would use that lesson's pronoun: "Kenen kassakaapin leski perii?"

What does the -kin suffix do at the end of leskikin?

The particle -kin means "also", "too", or "even". It attaches to the specific word it emphasizes. By attaching directly to the subject (leski), it means "The widow too" or "Even the widow" inherits the safe, implying someone else might be inheriting things as well.

What is the base form of the verb perii?

The base verb is periä (to inherit). It is a standard Type 1 verb. To form the third-person singular (he/she inherits), you take the stem peri- and double the final vowel, resulting in perii.