Ako skener opet ne radi, fotografirat ću dokument mobitelom i poslati ga kasnije.

Because Croatian normally uses the present tense after ako for a real future condition.

So:

• Ako skener opet ne radi = If the scanner doesn’t work again / if the scanner isn’t working again
• not usually a future form like Ako skener neće raditi

This is actually similar to English, where we also usually say If it doesn’t work, I’ll..., not If it won’t work, I’ll...

Raditi can mean both:

• to work = to do work / have a job
• to work = to function

Here it means to function.

So:

• Skener radi. = The scanner works / is working.
• Skener ne radi. = The scanner doesn’t work / isn’t working.

This is very common with machines and devices.

In Croatian, ne is usually written separately from the verb.

So:

• radi = works
• ne radi = does not work

A few forms are exceptions and are written together, such as nemam, nisam, and neću, but most verbs behave like ne radi.

Croatian often leaves out subject pronouns because the verb form already shows the person.

Here ću tells you the subject is I:

• fotografirat ću = I will photograph
• poslati is understood as part of the same future action

You could say ja ću, but that would usually add emphasis or contrast:

• Ja ću fotografirati... = I will photograph... (maybe stressing that I will do it)

Both are possible.

Croatian future tense can be formed in two common ways:

• Fotografirat ću dokument.
• Ću fotografirati dokument.

Well, more naturally:

• Fotografirat ću dokument.
• Fotografirat ću is the standard form when the infinitive comes before ću
• Ja ću fotografirati dokument or simply Ću fotografirati dokument is not normally used without something before ću, because clitics like ću usually do not stand at the very beginning of a neutral sentence

The important spelling point is this:

• infinitive: fotografirati
• before ću: fotografirat ću

When the infinitive comes before the future auxiliary, the final -i is dropped in writing.

The ću is understood for both verbs.

So this part means:

• fotografirat ću dokument mobitelom i poslati ga kasnije
• literally: I will photograph the document with my phone and send it later

Croatian often uses one future auxiliary for two coordinated infinitives.

If you wanted, you could repeat it, but you do not need to:

• Fotografirat ću dokument mobitelom i poslat ću ga kasnije.

The original version is smoother and very natural.

Because mobitelom is the instrumental singular of mobitel.

The instrumental case is often used to show the means or tool used to do something.

So:

• fotografirati dokument mobitelom = to photograph the document with a phone

This is the same idea as:

• pisati olovkom = to write with a pencil
• putovati vlakom = to travel by train

There is no preposition here; the case ending alone shows the meaning.

Dokument is in the accusative singular because it is the direct object of fotografirati.

However, dokument is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: dokument
• accusative: dokument

That is why there is no visible change.

Ga is the accusative singular clitic pronoun meaning him/it, and here it means it.

It refers back to dokument:

• poslati ga = send it

Since dokument is masculine singular, the pronoun is ga.

So the second part is literally:

• poslati ga kasnije = to send it later

Because poslati is the more natural choice for a single completed action: send it.

Croatian often distinguishes between:

• slati = imperfective, ongoing/repeated sending
• poslati = perfective, one complete sending action

In this sentence, the speaker means one future completed act: first photograph the document, then send it. So poslati fits better.

They are adverbs:

• opet = again
• kasnije = later

So:

• Ako skener opet ne radi = If the scanner doesn’t work again
• poslati ga kasnije = send it later

Opet is very common in everyday speech. A slightly more formal or neutral alternative is ponovno.

Because the sentence begins with a subordinate clause introduced by ako:

• Ako skener opet ne radi, ...

After that comes the main clause:

• fotografirat ću dokument mobitelom i poslati ga kasnije.

In Croatian, it is standard to separate that initial if-clause from the main clause with a comma.