Learn German: Der Hund ist schnell.

In a basic German statement, the verb must be the second idea in the sentence. Here, the subject Der Hund is the first idea (position 1), so the verb ist takes position 2.

The lesson teaches that "he/she/it" verb forms usually end in "-t". Is ist a regular verb?

No. The verb sein (to be) is highly irregular and doesn't follow the standard rules. Even though ist happens to end in the letter "t" for the "it/he" form here, you just have to memorize it as a special case.

Why doesn't the adjective schnell have an ending added to it?

When an adjective comes after a verb like "is" to describe the subject, it stays in its basic dictionary form. You only add endings to adjectives when they come directly in front of a noun (like "the fast dog").

Why does the sentence use Der instead of Das or Die?

In German, every noun has a grammatical gender that you have to memorize. Hund is a masculine noun, so it uses the masculine word for "the", which is der.