Learn Spanish (Spain): ¡Lava la lechuga verde, por favor!

Because it is a command! When you tell someone informally (using tú) to do something, you usually drop the final s. So instead of tú lavas (you wash), you say ¡lava! (wash!).

Why does verde come after lechuga instead of before it?

In Spanish, describing words (adjectives) almost always come right after the thing they describe. So instead of saying green lettuce, you say lettuce green (lechuga verde).

The word lechuga is feminine. Why does verde not change to end in an a?

Adjectives that end in an -e are gender-neutral. They stay the same whether they describe a masculine or feminine word. For example, it is el tomate verde (the green tomato) and la lechuga verde (the green lettuce).

What is the upside-down exclamation mark (¡) for?

In Spanish, questions and exclamations are framed with opening and closing punctuation marks. The upside-down mark (¡) warns you right away that an exclamation is starting, so you know exactly how to read it out loud.