Learn Finnish: Onneksi olkoon, tämä on suuri ilo. - Congratulations, this is a great joy.

It literally translates to 'let it be for luck'. Onneksi comes from the word onni (luck or happiness). Olkoon is a special command form of the verb olla (to be) that means 'let it be'. Even though the grammar of the second word is advanced, the whole expression is just learned early on as a fixed phrase.

Is onneksi the exact same word used for 'luckily' or 'fortunately'?

Yes! If you want to say 'Luckily it is sunny', you would also start with Onneksi. In both cases, it is the word onni (luck) combined with the -ksi ending, which describes a state, a role, or a result.

Why are we using the basic form suuri ilo instead of the partitive suurta iloa? Isn't joy an uncountable feeling?

Emotions are often uncountable and use the partitive case, but here the phrase describes a specific event or occasion. When you say 'this is a great joy', the joy is treated as a single, complete thing rather than an abstract continuous feeling. Because of this, it stays in the basic nominative case.

What is the difference between saying Onneksi olkoon and just saying Onnea?

Onnea is the most common, everyday way to say both 'congratulations' and 'good luck'. Onneksi olkoon is a fixed phrase that specifically means 'congratulations' (it is never used for 'good luck') and sounds a little more formal and festive.