Learn Spanish (Latin America): El árbol es verde y grande.

Adjectives that end in -e are invariable for gender, meaning they don't change to match masculine or feminine nouns. Words like verde and grande stay exactly the same whether they describe el árbol (masculine) or la planta (feminine).

How would this sentence change if there were multiple trees?

It would become Los árboles son verdes y grandes. To make adjectives ending in -e plural, you simply add an -s. Also notice that to make árbol plural, you add -es because it ends in a consonant.

Why do we use es instead of está here?

We use es (from the verb ser) because being green and big are considered basic, defining characteristics of this tree. The verb estar is used for temporary conditions or locations, but for physical descriptions and permanent traits, we always use ser.

Why is there an accent mark on árbol?

The accent mark shows you where to put the vocal stress when pronouncing the word. Spanish words ending in an l naturally stress the last syllable. The accent mark on the á breaks that normal rule, telling you to pronounce it as ÁR-bol instead of ar-BOL.