Umjesto taksija, idemo autobusom u grad.

Because umjesto normally takes the genitive case in Croatian.

• umjesto taksija = instead of a/the taxi
• base form: taksi
• genitive singular: taksija

So the structure is:

• umjesto + genitive

Other examples:

• umjesto kave = instead of coffee
• umjesto auta = instead of the car

Autobusom is the instrumental singular of autobus.

In Croatian, when you say you travel by some means of transport, you often use the instrumental case without a preposition:

• ići autobusom = to go by bus
• putovati vlakom = to travel by train
• doći autom = to come by car

So:

• idemo autobusom = we’re going by bus

This is one of the most important uses of the instrumental case.

Because Croatian usually expresses means of transport with the bare instrumental, without a preposition.

So:

• autobusom = by bus

But s autobusom would usually mean something more like:

• with the bus
• together with the bus
• accompanied by the bus

That is not the normal way to express transportation in this sentence.

So remember:

• Idemo autobusom. = We’re going by bus.
• not usually Idemo s autobusom.

Because Croatian uses different cases after u depending on whether there is movement or location.

1. u + accusative = movement into/to

• idemo u grad = we’re going into/to the city

2. u + locative = being in

• u gradu = in the city

So:

• Idemo u grad. = We are going to the city.
• Mi smo u gradu. = We are in the city.

This is a very common contrast in Croatian.

Idemo is the 1st person plural present tense of ići (to go).

So literally it means:

• we go
• we are going

In Croatian, the present tense is often used for something happening now or for a planned/near-future action, so in this sentence it naturally means:

• we’re going
• we’re going to go

Forms of ići in the present:

• idem = I go / I’m going
• ideš = you go
• ide = he/she/it goes
• idemo = we go
• idete = you go
• idu = they go

Croatian does not have articles like English a/an or the.

So a noun like taksi or grad can mean:

• a taxi / the taxi
• a city / the city

The exact meaning depends on context.

In this sentence:

• umjesto taksija could be understood as instead of a taxi or instead of the taxi
• u grad could mean to town / to the city

English needs articles, but Croatian usually does not.

Yes. Croatian word order is fairly flexible because case endings show the grammatical relationships.

This sentence:

• Umjesto taksija, idemo autobusom u grad.

could also be rearranged, for example:

• Idemo autobusom u grad umjesto taksija.
• U grad idemo autobusom umjesto taksija.

These versions are all understandable, but the emphasis changes.

The original order puts umjesto taksija first, which highlights the contrast:

• Instead of a taxi, we’re going by bus.

So the first position often gives special focus.

It is natural and helpful here because Umjesto taksija is an introductory phrase.

• Umjesto taksija, idemo autobusom u grad.

The comma marks a pause and makes the sentence easier to read.

In informal writing, some people might omit it, but with this fronted phrase, the comma is very standard and clear.

Here, grad is in the accusative singular.

The noun is:

• nominative: grad
• accusative: grad

For many masculine inanimate nouns in Croatian, the nominative and accusative singular have the same form, so you have to know the case from the preposition and meaning.

Because u here expresses movement, it requires the accusative:

• u grad = to the city

Compare:

• u gradu = in the city, locative

In sentences like this, umjesto functions as a preposition meaning instead of and it takes the genitive.

So here:

• umjesto taksija = instead of a taxi

You will most often meet it in exactly this kind of structure. For a learner, the key thing to remember is simply:

• umjesto + genitive

That will help you use it correctly in most everyday sentences.

A few natural alternatives are:

• Idemo u grad autobusom umjesto taksijem. — not correct
• Idemo u grad autobusom umjesto taksijem. — also not correct

The correct alternatives keep the same case pattern:

• Idemo u grad autobusom umjesto taksija.
• U grad idemo autobusom umjesto taksija.

Why not taksijem? Because after umjesto, Croatian wants the genitive, not the instrumental.

So the important pattern is:

• autobusom = instrumental, because it shows the means of transport
• taksija = genitive, because it follows umjesto

That contrast is exactly why the sentence is useful for learning cases.