Learn Spanish (Latin America): Mi padre cocina en el restaurante nuevo.

The verb cocinar (to cook) is a regular -ar verb. Because the subject is Mi padre (which counts as "he" or él), we drop the -ar and add the ending -a to make cocina. If you wanted to say "I cook," you would use the "I" ending and say cocino.

Why is it restaurante nuevo instead of nuevo restaurante?

In Spanish, descriptive adjectives like nuevo (new) almost always come right after the noun they describe. So, instead of saying "new restaurant," you literally say "restaurant new" (restaurante nuevo).

How do we know to use nuevo and not nueva?

Adjectives have to match the gender of the noun they describe. Restaurante is a masculine noun, which you can also tell by the masculine word for "the" (el) right before it. Therefore, we use the masculine adjective nuevo instead of the feminine nueva.

Does en mean "in" or "at" here?

It can mean either! The Spanish word en is very flexible and commonly translates to "in," "on," or "at." En el restaurante works perfectly whether you want to say "in the restaurant" or "at the restaurant."