Learn Dutch: De kat is daar. - The cat is there.

In Dutch, there are two words for 'the': de and het. Nouns belong to one of two groups based on gender, and you usually have to memorize which article goes with which word. Kat is a de-word, so it always takes de.

What is the difference between daar and hier?

Daar means 'there' (further away), while hier means 'here' (close to you). So de kat is daar means the cat is located somewhere away from the speaker.

Does the verb is change if I talk about multiple cats?

Yes! Is is the form of the verb zijn (to be) used for singular subjects like he, she, it, or de kat. If you were talking about multiple cats (de katten), you would use the plural form zijn (are), making it de katten zijn daar.