Learn Russian: К сожалению, магазин уже закрыт. - Unfortunately, the shop is already closed.

The phrase к сожалению literally translates as "toward regret". The preposition к always requires the dative case. Like the по- adverbs in this lesson, it acts as an introductory phrase expressing the speaker's attitude and must be separated from the rest of the sentence by a comma.

Why do we use закрыт here instead of the full adjective закрытый?

Закрыт is a short-form adjective. Russian often uses short forms to describe a temporary state or the current result of an action (the shop was closed and remains in that state). If you used the full form закрытый, it would sound like a permanent characteristic, such as a private or restricted-access shop.

Does the ending of закрыт change depending on what is closed?

Yes, short-form adjectives must agree with the subject in gender and number. Because магазин is masculine, we use the masculine form закрыт. If you were talking about a pharmacy (аптека), which is feminine, you would say аптека уже закрыта. For plural nouns, the ending is -ы (for example, двери закрыты).

Could I replace к сожалению with a word like по-моему from this lesson?

Absolutely! You could say По-моему, магазин уже закрыт (In my opinion, the shop is already closed). Both phrases serve the exact same grammatical role: they are introductory words that express the speaker's perspective, they sit outside the core grammar of the sentence, and they always need to be set off by a comma.