Learn Portuguese (Portugal): A montanha é tão alta como a cidade?

To say something is "as [adjective] as" something else, Portuguese uses the pattern tão + adjective + como. Here, tão translates to the first "as", alta is the adjective ("high"), and como translates to the second "as".

Why does the adjective alta end in an "-a"?

Adjectives in Portuguese have to match the gender of the noun they describe. Since montanha (mountain) is a feminine noun, the masculine adjective alto changes to the feminine alta.

Why do we use é instead of está for the mountain's height?

We use é (from the verb ser) because the height of a mountain or city is a permanent characteristic. You would only use está (from the verb estar) for temporary states or conditions.

I have seen como used to mean "how" or "I eat". How do I know it means "as" here?

Context is key! While como is the question word for "how" and the "I" form of the verb comer (to eat), in this sentence it is paired with tão (tão alta como). This tão... como combination is the standard way to compare two things equally.