Learn Turkish: Yatakta uyumak çok iyi. - Sleeping in bed is very good.

In Turkish, the dictionary form of a verb ending in -mak or -mek acts as a noun. While we often translate it as "to sleep", it is also used where English uses "-ing" words as the subject of a sentence. So, uyumak means both "to sleep" and the act of "sleeping".

If English uses "-ing" for "sleeping", could I use the present continuous form uyuyor instead?

No, because uyuyor is an active verb meaning "is sleeping" (like "He is sleeping"). In our sentence, "sleeping" is a concept acting as the subject. Whenever an action is the subject of your sentence, you must use the -mak or -mek infinitive form.

If the suffix for "in" is -da, why is the word written as yatakta instead of yatakda?

This is due to consonant harmony. The word yatak (bed) ends with k, which is a hard (voiceless) consonant. To make pronunciation flow better, the soft d in the suffix -da changes to a hard t, becoming -ta.

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

Turkish does not have a separate word for "is" in simple present tense descriptions. Simply placing çok iyi (very good) after the subject makes a complete sentence meaning "is very good".