Learn Dutch: Er is een klein restaurant op het plein. - There is a small restaurant on the square.

Er is translates exactly to 'there is' in English. We use it to introduce something new that exists in a particular place. Because een klein restaurant is singular, we use is. If we were talking about multiple restaurants, we would use er zijn (there are).

Why does the adjective not have an -e at the end (klein instead of kleine)?

Normally, adjectives get an -e at the end when placed before a noun. However, restaurant is a het word. When a het word is singular and comes after the word een (a/an), the adjective loses its -e. That is why it is een klein restaurant.

Why do we use the preposition op for the square?

In Dutch, open public spaces like a square (het plein), a street (de straat), or a market (de markt) are treated as flat surfaces. So, a building or a person is placed op (on) the square. English sometimes uses 'in the square', but Dutch strictly prefers op here.