Learn Finnish: Orkideani tarvitsee lannoitetta kolme kertaa kuukaudessa. - My orchid needs fertilizer three times a month.

When expressing frequency, Finnish uses the inessive case (the -ssa or -ssä ending, which usually means 'in'). So 'three times a month' is formed literally as 'three times in a month' (kolme kertaa kuukaudessa). You use this exact same pattern for other time periods, like viikossa (in a week) or vuodessa (in a year).

Why is the word for 'time' in the partitive case (kertaa)?

In Finnish, any number greater than one requires the following noun to be in the partitive singular case. Because we are saying kolme (three), the base word kerta (time) must change to its partitive form, kertaa.

Why is the fertilizer (lannoitetta) in the partitive case?

The verb tarvita (to need) takes an object in the partitive case when referring to an uncountable mass noun or an indefinite amount. The orchid needs 'some fertilizer' rather than one specific, indivisible unit of it, so the base word lannoite gets the partitive -tta ending.

How does the base word kuukausi (month) turn into kuukaudessa?