Learn Turkish: Ağaçlar büyük.

This is because of the 2-way vowel harmony rule. We look at the last vowel in ağaç, which is the letter a. Because it is a back vowel, the plural suffix must also use a back vowel, making it -lar.

Where is the word for are in this sentence?

Turkish does not have a separate word for am, is, or are. In simple descriptive sentences, you just place the subject and the adjective together. Ağaçlar büyük literally means Trees big, which translates to The trees are big.

Should the adjective büyük also be plural to match ağaçlar?

No, in Turkish, adjectives stay in their singular form even when they describe plural nouns. You only need the plural suffix on the noun itself.

How do I pronounce the ğ in ağaçlar?

The letter ğ is called a soft g and it is usually silent. Instead of making its own sound, it simply lengthens the vowel right before it. You can pronounce the first part of the word like aah.