Learn Dutch: De volle bussen zijn laat. - The full buses are late.

Because the adjective is placed directly in front of the noun it describes (bussen). When an adjective is used this way, we almost always add an -e to the end of it.

Why does vol change to volle with two Ls?

To keep the short 'o' sound! The word vol has a short vowel. If we just added an -e to make vole, the rules of Dutch pronunciation would make it sound like a long 'o' (voh-le). We double the 'l' to protect that short sound. You can see the exact same rule happening when bus becomes bussen.

Why doesn't laat get an -e at the end like volle does?

Because laat comes after the verb (zijn), rather than directly before a noun. In Dutch, we only add the extra -e ending when the adjective is placed right in front of the noun it describes.

Would it still be volle if we were only talking about one bus?

Yes! The singular form is de bus, which makes it a de word. Adjectives directly in front of de words always get the -e ending, whether they are singular or plural.