El imprevisto cambia el reparto de tareas.

Because el imprevisto is a singular masculine noun phrase meaning something like the unexpected event, the unforeseen problem, or the unexpected situation.

In Spanish, nouns usually need an article much more often than in English. So where English might say Unexpected trouble changes..., Spanish naturally says El imprevisto cambia...

Here, el agrees with imprevisto, which is treated as masculine singular.

Here, imprevisto is being used as a noun.

As an adjective, it can mean unforeseen or unexpected, for example:

• un problema imprevisto = an unforeseen problem

But in your sentence, el imprevisto works as a noun, meaning:

• the unforeseen event
• the unexpected issue
• the unexpected circumstance

This is common in Spanish: an adjective can become a noun when used with an article.

Because the subject is el imprevisto, which is singular.

So the verb must also be singular:

• el imprevisto cambia = the unexpected event changes

If the subject were plural, then you would use cambian:

• los imprevistos cambian el reparto de tareas = unexpected events change the distribution of tasks

Cambia is the third-person singular present indicative of cambiar.

So it means:

• changes
• or sometimes is changing, depending on context

In this sentence, it is most naturally understood as a general present:

• The unexpected event changes the distribution of tasks

Because reparto is also a noun, and here it refers to a specific thing: the distribution/allocation/division of tasks.

So:

• el reparto = the distribution / the allocation

Spanish often uses the definite article where English may or may not use the, depending on style.

In this sentence, reparto means the division, allocation, or distribution of tasks among people.

It comes from the verb repartir, meaning to distribute, to hand out, or to divide up.

So el reparto de tareas is something like:

• the distribution of tasks
• the allocation of tasks
• the division of duties

A useful nuance: reparto often suggests that something is being shared out among different people.

Because de tareas is a very common and natural Spanish structure meaning of tasks in a general sense.

So:

• el reparto de tareas = the distribution of tasks

If you said de las tareas, it would usually sound more specific, like you mean the tasks already known in the conversation:

• el reparto de las tareas = the distribution of the tasks

Both are possible, but de tareas is more general and idiomatic here.

Sometimes, yes, depending on context, but not always.

El reparto de tareas usually refers directly to who does which tasks. It is often more concrete and everyday than division of labour.

Possible translations include:

• distribution of tasks
• allocation of tasks
• division of tasks
• sharing out of duties

Division of labour can work in some contexts, but it can sound a bit more formal or abstract in English.

Yes, you could say that, but it changes the focus.

Your original sentence:

• El imprevisto cambia el reparto de tareas

focuses on the unexpected event as the thing causing change.

If you say:

• El reparto de tareas cambia por el imprevisto

the focus shifts more toward the distribution of tasks itself, and then you add the cause: because of the unexpected event.

Both are grammatical, but they are not exactly the same in emphasis.

Not exactly by itself, though in some contexts it might refer to something that causes a delay.

Imprevisto is broader than delay. It means:

• an unforeseen event
• an unexpected problem
• a last-minute issue

So it could be a delay, a technical problem, an absence, bad weather, or anything unexpected.

Both are possible, but they mean slightly different things.

• El imprevisto cambia el reparto de tareas suggests a specific unexpected event, or it can sound more generic in a formal or explanatory way.
• Un imprevisto cambia el reparto de tareas means an unexpected event changes the distribution of tasks, which sounds more indefinite and general.

Spanish often uses el in cases where English learners might expect un.

Yes, it sounds natural.

In Spain, reparto de tareas is a normal and idiomatic expression for talking about how tasks are divided. Imprevisto is also very common in everyday and formal Spanish.

The sentence has a neutral, standard tone and would be understood easily in Spain.

The stress is:

• impreVISTo
• rePARto

More roughly for an English speaker:

• eem-preh-BEES-toh in much of Spain, where v and b sound very similar
• reh-PAR-toh

And the full sentence is stressed roughly like this:

• El impreVISTo CAMbia el rePARto de TAreas

If you are learning Spain Spanish, remember that c before i in cambia is a normal k sound, because it is ca, not ce/ci.