Learn Finnish: Aineellinen turva on tärkeää, itseilmaisu taas tuo onnellisuutta. - Material security is important, whereas self-expression brings happiness.

In this context, taas doesn't mean 'again' but acts as a connector of contrast meaning 'whereas' or 'on the other hand'. When used this way, it has an enclitic-like placement: it doesn't start the clause, but instead tucks in immediately after the subject or topic it is contrasting. Here, it highlights the contrast between aineellinen turva and itseilmaisu.

Can I just use mutta (but) instead of the taas structure here?

Yes, you could say , mutta itseilmaisu tuo..., but taas serves a slightly different purpose. While mutta introduces a direct contradiction or exception, this placement of taas creates a balanced comparison between two parallel facts, much like 'whereas' or 'while' in English.

I always mix up when to use the partitive for adjectives. Why is it tärkeää instead of the nominative tärkeä?

The subject here, aineellinen turva (material security), is an abstract, uncountable concept. In Finnish, when you have an 'A is B' sentence and the subject is uncountable, the predicative adjective describing it must be in the partitive case, hence tärkeää.

Why does the verb tuo take the partitive object onnellisuutta?

Onnellisuus (happiness) is an abstract noun. Because bringing happiness is an ongoing, open-ended result rather than a completed action with a concrete, countable object, it requires the partitive case.