Learn German: Die Häuser und die Bäume sind auch groß. - The houses and the trees are also big.

In German, the definite article for all plural nouns is die, no matter what gender the noun has in the singular. So, das Haus (neuter) becomes die Häuser, and der Baum (masculine) becomes die Bäume.

How do Haus and Baum change to become plural?

Both words add an umlaut to their vowel (the a becomes ä), but they take different endings. Haus adds -er to become Häuser, while Baum adds just an -e to become Bäume. German has a few different ways to make plurals, so it is usually best to memorize the plural form when you learn a new noun.

The letters "au" change to "äu" in both plural words here. How does that change the pronunciation?

The plain au in Haus and Baum sounds like the "ou" in the English word "house". However, when you add the umlaut, the äu combination in Häuser and Bäume sounds exactly like the "oy" in the English word "boy".

Since the nouns are plural, why doesn't the adjective change to große?