Learn Turkish: Okyanuslar Dünya'yı kaplar.

The root of the verb is kapla- (from the infinitive kaplamak, meaning "to cover"). Because we are using the aorist (simple present) tense for a general fact, and the root ends in a vowel, we simply add the suffix -r to get kaplar.

The subject Okyanuslar (oceans) is plural. Why doesn't the verb have a plural ending, like kaplarlar?

In Turkish, when a plural subject is not human (like animals, objects, or concepts), the verb usually remains in the singular form. Since oceans are non-human, Okyanuslar ... kaplar is the correct phrasing.

Why does Dünya'yı have an apostrophe and the -yı ending?

Dünya (Earth) is treated as a proper noun here, so any suffixes added to it must be separated by an apostrophe. The -yı is the accusative (specific direct object) suffix, made of the buffer letter -y- and the vowel -ı. We use the accusative case because the Earth is the specific thing being covered by the oceans.

Why use the aorist tense (kaplar) here instead of the present continuous (kaplıyor)?

The aorist tense is the go-to tense for stating general facts, scientific truths, and habits. Since "oceans cover the Earth" is a permanent fact of nature rather than a temporary action happening right at this moment, the aorist is the most natural choice.