Learn Finnish: Minä valitsin numeron, koska salasana ei ole sana. - I selected a number, because the password is not a word.

Valita is a Type 5 verb. To form the past tense, you first find the present tense stem by dropping the -a and adding -se-, giving us valitse-. Then, for the past tense, the -e- changes to -i-, making the past stem valitsi-. Finally, add the personal ending -n for "I", resulting in valitsin.

Why is numero written as numeron with an -n at the end?

Numeron is the direct object of the verb valitsin. Because you completed the action of choosing one specific number, the object takes the genitive-accusative ending -n.

Is salasana a compound word?

Yes! It is put together from sala (secret) and sana (word). So a "secret word" translates perfectly to a password.

Negative sentences usually force words into the partitive case. Why is sana not in the partitive?

That rule applies to direct objects, but here sana is a predicative (it just states what the subject is). With the verb olla (to be), a singular noun stays in its basic nominative form, even in negative sentences like ei ole sana.