Learn German: Der Wald hier ist älter als der Park. - The forest here is older than the park.

In English, you add "-er" to make a comparison (old to older). German does the same, but many short, one-syllable adjectives with an "a", "o", or "u" also get an umlaut in their comparative form. So alt adds an "-er" and an umlaut to become älter.

What does als mean in this sentence?

Here, als means "than". You use als whenever you are comparing two things that are different, like one being older than the other. If the forest and the park were the exact same age, you would use wie instead (meaning "as"), like in so alt wie (as old as).

Why is it der Park and not den Park?

When you compare two things using als, they both stay in the same grammatical case. Because Der Wald is the subject of the sentence (Nominative case), der Park must also be in the Nominative case.

Why does hier come right after Der Wald?

In this sentence, Der Wald hier acts as a single block meaning "The forest here". If you moved hier to the end of the sentence (Der Wald ist älter als der Park hier), it would change the meaning to "The forest is older than the park here"!