Learn Croatian: Nedjeljom idemo u crkvu. - On Sundays we go to church.

Nedjeljom is the instrumental case of the word nedjelja (Sunday). In Croatian, using the instrumental case for a day of the week means the action happens regularly. It translates to "on Sundays" or "every Sunday". If you wanted to talk about a specific single Sunday, you would use u nedjelju.

Why is the word for church crkvu here instead of its dictionary form crkva?

The noun crkva is in the accusative case here. When the preposition u is used with a verb of motion like ići (to go), it requires the accusative case to show the destination you are moving towards. For feminine nouns ending in -a, the ending changes to -u.

How would the sentence change if we were already inside the church?

If you want to talk about a stationary location rather than movement toward a destination, the preposition u takes the locative case. To say "We are in the church", you would say Mi smo u crkvi. The ending changes from -a to -i.

Is it okay to put nedjeljom at the end of the sentence?

Yes, Idemo u crkvu nedjeljom is completely grammatically correct. Croatian has a very flexible word order. Placing nedjeljom at the beginning of the sentence just puts a little more emphasis on the day.