Learn German: Herzlich willkommen! Das Haus ist groß.

In German, willkommen means "welcome". Adding herzlich makes it warmer and more polite. Herzlich literally means "cordial" or "hearty" (from Herz, meaning "heart"). So you are wishing someone a hearty welcome!

Why do we use das before Haus?

In German, all nouns have a grammatical gender: masculine (der), feminine (die), or neuter (das). Haus is a neuter noun, so it takes the neuter word for "the", which is das.

What is the letter that looks like a "B" in groß?

That is the letter ß (called an Eszett or sharp S). It is unique to German and makes a sharp "s" sound, like the "s" in the English word "snake". If you do not have a German keyboard, it is perfectly acceptable to type it as a double "s" (gross).

Why doesn't groß have an extra letter at the end?

Because it comes after the noun it describes (das Haus). In German, adjectives only get special endings when they are placed directly before the noun. When they follow a verb like ist, they stay in their basic, dictionary form.