Learn Croatian: Došli smo taksijem do muzeja. - We came to the museum by taxi.

When using the instrumental case to describe a means of transport or a tool, you do not use the preposition s or sa. If you said s taksijem, it would mean you arrived in the company of a taxi, as if the taxi were a person arriving alongside you!

Where does the j in taksijem come from?

The base noun is taksi. Because it is a masculine loanword ending in the vowel i, Croatian inserts a j before the standard instrumental ending -em to make the transition between the vowels easier to pronounce.

Why use do muzeja instead of u muzej?

Using the preposition do with the genitive case (muzeja) means you traveled up to or as far as the museum. It focuses on the route's endpoint. If you used u muzej (which requires the accusative case here), it would specifically emphasize going inside the building.

Could we use dolazili smo instead of došli smo?

Usually no, not if you are describing a single completed trip. Došli comes from the perfective verb doći, which shows the action of arriving is totally finished. Dolazili is the imperfective form, which you would only use to mean you were in the process of coming or that you used to come repeatedly.