Learn Portuguese (Portugal): Estas calças são muito feias.

We use the este family of words for things that are close to the speaker. Since the speaker is talking about pants right next to them (or wearing them), we use Estas (these). If the pants were near the person being spoken to, we would use Essas (those). Both are feminine and plural to match calças.

Is the word calças always plural, just like in English?

Yes! Just like we say pants or trousers in English, calças is treated as a plural word in Portuguese. That is why all the connected words are also in the plural form: Estas, são, and feias.

Since calças is feminine and plural, why doesn't muito change to muitas?

When muito means very and comes before an adjective (like feias), it is an adverb. Adverbs never change their shape. You only change it to muitas when it means many or a lot of and comes directly before a noun, such as muitas calças (many pants).

Why use são (from the verb ser) instead of estão (from the verb estar)?