Learn Spanish (Latin America): Esta montaña es más alta que el valle.

In Spanish, you don't add "-er" to the end of an adjective like we often do in English. Instead, you put más (more) before the adjective and que (than) after it. So, "higher than" translates to más alta que (literally: "more high than").

Why is the adjective alta (feminine) if we are comparing it to el valle (masculine)?

In a comparison, the adjective always matches the subject of the sentence, not the thing it is being compared to. Because the subject is Esta montaña (a feminine noun), the adjective must be the feminine alta.

Why do we use es instead of está to describe the mountain?

We use es (from the verb ser) because the height of a mountain is a permanent, inherent physical characteristic. We use estar for temporary states or locations, but to describe what a mountain is fundamentally like, ser is the correct choice.

Why do we use Esta for "This" instead of Este?

Montaña is a feminine noun, so "this mountain" must be Esta montaña. If we were talking about a masculine noun instead, like el valle, we would use Este (for example, Este valle).