Learn Turkish: En önemli dosya eski. - The most important file is old.

The word en means 'most' and it simply goes directly before the adjective it modifies. Unlike English, which sometimes changes the word (like 'biggest') and sometimes adds 'most', Turkish always just puts en in front of the adjective. So en önemli is 'the most important'.

Where is the word for 'is' in this sentence?

There isn't a separate word for 'is' here. In Turkish, when talking about 'he', 'she', or 'it', the 'is' is usually invisible. Simply putting the adjective eski at the very end of the sentence turns it into the main action, giving us 'is old'.

Could I put eski before dosya instead?

That would change the meaning completely! If you said en önemli eski dosya, it would mean 'the most important old file'. By keeping eski at the end, separated from the noun, it acts as the sentence's main point (the predicate): 'the most important file is old'.

I remember seeing the suffix -li on other words. Is that what is happening in önemli?

Yes, exactly! The suffix -li adds the meaning of 'with' or 'having' to a noun. The base word here is önem, which means 'importance'. Adding -li makes önemli, which literally means 'having importance', or simply 'important'.