Learn Croatian: Vidim jedan restoran.

In Croatian, you often drop subject pronouns like ja (I) because the verb ending already tells us who is doing the action. The -m at the end of vidim clearly means 'I see'.

Does the number jedan (one) change depending on what I am counting?

Yes. Unlike English, the number one in Croatian acts like an adjective, so it changes to match the gender of the noun. Because restoran is a masculine word, we use the masculine form jedan.

If jedan restoran is the object of 'I see', why don't the endings change?

They actually are in the object form (called the accusative case). But there is a special rule for masculine nouns: if the object is non-living (like a building), its object form looks exactly the same as its dictionary form. The same goes for the number jedan attached to it.

Can jedan be used to mean 'a' or 'an', like 'a restaurant'?

Croatian does not use words like 'a', 'an', or 'the'. If you just want to say 'I see a restaurant', you would normally just say Vidim restoran. Using jedan specifically emphasizes the number: you see exactly one restaurant, not two.