Learn Turkish: Bu söz çok önemli. - This word is very important.

While both can translate to 'word,' they are used differently. Kelime refers to a structural vocabulary word, like the words you look up in a dictionary. Söz refers more to a spoken remark, a saying, or a promise (like 'I give you my word'). In this sentence, it means 'this statement' or 'this remark' is very important.

I see the suffix -li on önemli. How does that work?

The suffix -li (and its vowel harmony variations -lı, -lu, -lü) means 'with' or 'having.' The root word here is önem, which means 'importance.' By adding -li, you create önemli, which literally means 'having importance'—the Turkish way of saying 'important'.

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

Turkish doesn't use a separate word for 'is' (the 'to be' verb). Instead, it uses personal suffixes attached to the final word. For 'he/she/it' subjects, this suffix is usually left completely blank in everyday speech, just like in this sentence. You could technically add the suffix -dir to make it önemlidir, but leaving it off is very common and sounds natural.