Learn Finnish: Sää oli eilen ihana. - The weather was wonderful yesterday.

oli is the past tense (imperfect) of the verb olla (to be) for the he/she/it form. Because the sentence happens in the past (eilen meaning yesterday), we use oli (was) instead of the present tense on (is).

How would I say the weather wasn't wonderful yesterday?

To make the past tense of olla negative for a singular subject like the weather, you use the negative word ei plus ollut. The negative sentence would be: Sää ei ollut eilen ihana.

Does the time word eilen have to go in the middle of the sentence?

No, Finnish word order is quite flexible! You could also say Eilen sää oli ihana if you want to put a little more focus on the fact that it was yesterday. The order in our example is just a very natural, neutral choice.

I sometimes see the word ilma used for weather. Could I use that instead of sää?

Yes, you can! While ilma literally translates to air, it is extremely common to use it to mean weather in everyday speech. Ilma oli eilen ihana would sound perfectly natural to a Finn.