Learn Dutch: Vier paarden lopen buiten. - Four horses are walking outside.

You just add the regular -en suffix to the singular word paard to make paarden (horses). This is the most common way to make plural nouns in Dutch.

I remember that paard sounds like it ends in a 't'. Does that sound change in paarden?

Yes! In Dutch, words never end in a 'd' sound, so the singular paard is pronounced with a 't' at the end. But when you add -en to make the plural paarden, the 'd' is no longer at the end of the word, so it goes back to sounding like a normal 'd'.

Why does the verb lopen end in -en here instead of being loopt?

Because the subject, vier paarden (four horses), is plural. In Dutch, plural subjects take the plural form of the verb. For almost all verbs, the plural form is exactly the same as the dictionary form, which ends in -en.

The English translation says "are walking", but is there a word for "are" in the Dutch sentence?

No, because Dutch doesn't usually use extra words like "are" or "is" to show that an action is happening right now. The single Dutch verb lopen translates to both "walk" and "are walking".