Learn Turkish: Bu masa yuvarlak. - This table is round.

Because it isn't modifying the noun to say "a round table" (yuvarlak masa). Instead, it's the main point of the sentence: "This table is round." When an adjective is the main fact you are stating about something, it goes at the end of the sentence.

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

Turkish usually leaves out the word for "is" in simple, everyday statements about "he," "she," "it," or "this." You just state the subject (bu masa) and the describing word (yuvarlak), and the "is" is naturally understood.

Does bu count as an adjective placed before the noun here?

Yes! Bu (this) acts as a demonstrative (pointing) adjective in this sentence. Just like descriptive words, it must be placed directly before the noun it specifies, creating bu masa (this table).

How would the Turkish change if I wanted to say "This is a round table"?

You would place the adjective in front of the noun to modify it directly: Bu, yuvarlak bir masa. Notice how yuvarlak (round) now comes right before the noun, exactly following the rule for adjectives modifying nouns!