Learn Finnish: Minä luen usein runoja.

The basic word is runo (poem). To make it partitive plural, we simply add the ending -ja. Since runo contains the back vowels u and o, vowel harmony dictates we use -ja instead of -jä. Words ending in -o don't change their stem before this ending, so it attaches directly.

Why is "poems" in the partitive plural (runoja) instead of the regular plural (runot)?

In Finnish, the object of a sentence takes the partitive case when the action is ongoing, habitual, or involves an unspecified amount. Because you are reading an undefined number of poems on a regular basis (usein), the object must be in the partitive plural. If you read a specific, finished set of poems, you would use the regular plural object form.

The dictionary form of "to read" is lukea. Why is it luen here without the k?

This is due to consonant gradation, a rule where certain consonants change when grammatical endings are added. When we add the "I" ending (-n) to lukea, it closes the syllable. This forces the strong consonant k to disappear into its weak grade, changing the stem from luke- to lue-, resulting in luen.

In English we say "I often read", but here it is Minä luen usein. Is this word order required?