Learn Dutch: Het kind loopt snel naar de hond. - The child walks quickly to the dog.

This is a great example of the open and closed syllable spelling rules. In the full verb lopen (to walk), the first syllable is open (lo-pen), so a single 'o' already makes a long sound. When we chop off the -en to find the verb stem, we are left with a closed syllable. To keep that long 'o' sound, we have to double the vowel to make loop, and then we add the -t for the "het kind" form to get loopt.

Both kind and hond end in 'd'. Are they pronounced like the English 'd'?

No, in Dutch, whenever a 'd' is at the very end of a word, it is pronounced like a 't'. So kind sounds like "kint" and hond sounds like "hont". You only hear the true 'd' sound when the word gets longer and the 'd' starts a new syllable, such as in the plural form honden.

Why does the sentence use het for kind but de for hond?

Dutch has two different words for "the": de and het. Every noun belongs to one of these two categories, and you mostly just have to memorize which article goes with which word. Kind (child) happens to be a "het-word" and hond (dog) is a "de-word".

Why doesn't the word snel get a special ending like the English adverb "quickly"?