Learn Dutch: Er is een groot museum in deze stad. - There is a large museum in this city.

Just like in English, we use Er is (There is) to introduce a new, non-specific thing into the conversation. Dutch rarely starts a sentence with Een (A/An) plus a subject; instead, it prefers to use Er at the beginning to point out that something exists.

Why is it groot museum and not grote museum?

Adjectives usually get an -e added to the end. However, museum is a het word. When a het word is used with een (a/an) rather than het, the adjective does not get the extra -e. That is why it remains groot.

Why is it deze stad and not dit stad?

Because stad (city) is a de word. When you want to say "this" with a de word, you must use deze. You only use dit with het words.

What is the actual subject of this sentence?

The real subject is een groot museum. Even though Er is sitting in the first position, it is just a placeholder. Because Er is in spot one, the verb is must take spot two, and the real subject is pushed to spot three.