Learn Turkish: Bence bu olgunca bir karar değil. - In my opinion, this is not a mature decision.

The word bence is made of the pronoun ben (I) and the suffix -ce. This suffix is often used to express a point of view or perspective. So, bence translates to "according to me" or "in my opinion". You can also add it to sen (you) to make sence (in your opinion).

What is the -ca doing at the end of olgunca?

It is the same suffix as in bence, but here it takes the form -ca because of vowel harmony with the adjective olgun (mature). When attached to an adjective, it means "rather", "quite", or "in a [adjective] way". Here, olgunca means "rather mature" or "mature-like".

Why is the word bir placed after olgunca instead of before it?

In Turkish, the indefinite article bir (a/an) almost always goes between the adjective and the noun. While in English we say "a mature decision," in Turkish the order is Adjective + bir + Noun, giving us olgunca bir karar.

Why is the sentence negated with değil instead of a negative verb suffix?