Learn Turkish: Bu yıkılmış bina artık sadece bir harabe. - This destroyed building is just a ruin now.

It is formed from the verb yıkmak (to destroy). First, the passive suffix -ıl is added to make yıkılmak (to be destroyed). Then, the past participle suffix -mış is added to turn it into an adjective describing the building. So, yıkılmış bina literally means 'the having-been-destroyed building.'

I thought artık meant 'anymore.' Why does it translate to 'now' here?

You usually see artık translated as 'anymore' when it is used with negative verbs. However, in positive sentences like this one, it means 'now' or 'from now on.' It specifically emphasizes that a change of state has happened: it used to be a normal building, but now it is a ruin.

Why is it sadece bir harabe instead of bir sadece harabe?

Just like in English where we say 'just a ruin,' the adverb sadece (only/just) comes before the indefinite article bir. Adverbs that modify the whole noun phrase generally go before bir.

Why isn't there a verb or suffix at the very end of the sentence for 'is'?

In Turkish, the third-person 'to be' suffix (-dır or -dir) is often dropped in everyday spoken and written language. You could write sadece bir harabedir, but simply ending with the noun harabe is perfectly natural and correct.