Ako na košulji opet visi jedan gumb, neću je obući.

After ako meaning if, Croatian very often uses the present tense to express a condition, just like English does in sentences such as If the button is loose again, I won’t wear the shirt.

So:

• Ako ... visi ... = If ... is hanging / is loose ...
• neću je obući = I won’t put it on

The present here does not mean only right now. It describes the condition that must be true for the main clause to happen.

Košulji is in the locative singular.

The preposition na can take different cases:

• na + locative = location, on / at
• na + accusative = movement onto something

Here there is no movement. The button is already on the shirt, so we get:

• na košulji = on the shirt

Compare:

• Gumb je na košulji. = The button is on the shirt.
• Stavio sam gumb na košulju. = I put the button onto the shirt.

Visi is the 3rd person singular present of visjeti or visiti, meaning to hang.

In this sentence, it does not mean that the button is neatly attached and simply located on the shirt. It suggests that the button is dangling, hanging loose, probably attached by only a thread or almost falling off.

So jedan gumb visi is a natural way to say that a button is loose and hanging.

Very often, yes. Since Croatian has no articles, jedan can sometimes help express the idea of a / one.

So jedan gumb can mean:

• one button in the literal numeric sense, or
• a button, introducing it as an indefinite item

In this sentence, it most naturally means something like one of the buttons / a button.

You could sometimes omit jedan and just say Ako na košulji opet visi gumb..., but jedan gumb sounds a bit more specific and natural here.

Neću is the negative form of hoću.

• hoću = I want / I will
• neću = I won’t

In Croatian, the future is often formed with forms of htjeti plus the infinitive:

• obući ću = I will put on
• neću obući = I will not put on

In the negative, neću is written as one word, not separately.

So:

• neću je obući = I won’t put it on

Je refers to košulju meaning the shirt.

It is an unstressed personal pronoun in the accusative singular feminine.

Why accusative? Because obući here takes a direct object: you put on something.

So:

• košulja = shirt
• košulju = shirt, as a direct object
• je = it, referring back to that feminine noun

That is why je means it here: I won’t put it on.

Because je is a clitic, an unstressed little word. Croatian clitics normally have a fixed position and do not usually go at the end like neću obući je.

So the natural order is:

• neću je obući

Here je comes before the infinitive obući.

A useful thing to remember is that short unstressed pronouns in Croatian usually stand early in the clause, in their normal clitic position.

If you want emphasis, you would normally use a different, stressed form, for example:

• Neću obući nju, nego drugu. = I won’t put on that one, but another one.

Obući is a perfective verb and means to put on / dress oneself in.

That fits this sentence because the speaker is talking about a single completed action: the decision not to put the shirt on.

Compare:

• obući = to put on, one complete act
• oblačiti = to be putting on / to put on repeatedly, imperfective
• nositi = to wear, in the sense of having something on

So:

• Neću je obući. = I won’t put it on.
• Neću je nositi. = I won’t wear it.

Both can work in some contexts, but obući focuses on the act of putting the shirt on, which is very natural here.

Yes, in many contexts you can.

Both je and ju can be used as unstressed accusative feminine singular pronouns meaning her / it.

So these are both possible:

• Neću je obući.
• Neću ju obući.

In real usage, preferences vary by region, style, and speaker. In this sentence, je is completely normal.

Croatian word order is fairly flexible, but the original sentence sounds natural and neutral.

Original:

• Ako na košulji opet visi jedan gumb, neću je obući.

You could also hear variants such as:

• Ako opet na košulji visi jedan gumb, neću je obući.
• Ako jedan gumb opet visi na košulji, neću je obući.

The basic meaning stays the same, but the emphasis changes slightly.

The original order puts the shirt and its condition first, which works well because that condition is what motivates the decision in the second clause.