El imprevisto cambia la estimación.

In Spanish, singular countable nouns usually need an article or another determiner.

So imprevisto on its own would normally sound incomplete here. You usually say:

• el imprevisto = the unforeseen issue / the unexpected event
• un imprevisto = an unforeseen issue / an unexpected event

Using el suggests that the speaker has a specific unexpected issue in mind, or is speaking about it as something already identified in the context.

That is a very common question. Imprevisto can be:

• an adjective: un gasto imprevisto = an unexpected expense
• a noun: un imprevisto = an unforeseen event / an unexpected problem

In this sentence, imprevisto is a noun.
The article el helps show that:

• el imprevisto = the unexpected issue/event

This is very natural in Spanish.

Because estimación is a feminine noun.

A very common pattern in Spanish is that nouns ending in -ción are feminine:

• la estimación
• la situación
• la información
• la decisión

So the article has to agree with the noun:

• la estimación

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

So it matches a singular subject such as:

• el imprevisto cambia
• la situación cambia
• el precio cambia

In this sentence, the subject is el imprevisto, so the verb is singular: cambia.

Because Spanish verb endings work differently from English.

In English, he/she/it changes adds -s.
In Spanish, the verb itself changes form according to the subject:

• yo cambio = I change
• tú cambias = you change
• él/ella cambia = he/she changes

So cambia already means changes when the subject is he/she/it or a singular noun like el imprevisto.

Because cambiar can take a direct object directly in Spanish.

Here:

• El imprevisto = subject
• cambia = verb
• la estimación = direct object

Spanish does not need a preposition here.

Compare:

• Cambia la fecha. = It changes the date.
• Cambia el resultado. = It changes the result.
• Cambia la estimación. = It changes the estimate.

The normal, neutral order is:

Subject + verb + object
El imprevisto cambia la estimación.

That is the most straightforward version.

Spanish can sometimes change word order for emphasis, but this sentence in its original form sounds neutral and natural. For example, Cambia la estimación el imprevisto would sound marked or literary in most contexts.

Often yes, but the exact nuance depends on context.

Estimación can refer to:

• an estimate
• an approximation
• an assessment

In business, technical, or planning contexts, it can be very close to estimate.
In other contexts, Spanish might also use words like:

• cálculo = calculation / estimate
• presupuesto = budget / quote
• valoración = assessment / evaluation

So estimación is correct, but the best English equivalent depends on the situation.

Grammatically, yes, but it means something slightly different.

• El imprevisto cambia la estimación = the unexpected issue changes the estimate
• Un imprevisto cambia la estimación = an unexpected issue changes the estimate

So the difference is the same kind of difference as the vs an in English:

• el = specific / identified
• un = nonspecific / any one example

The accent mark shows where the stress goes:

es-ti-ma-CIÓN

The stress falls on the last syllable: -ción.

Without the accent mark, Spanish spelling rules would make you stress it differently, so the written accent is important.

In most of Spain, the c before i is pronounced like th in think, so many speakers in Spain say something close to:

es-ti-ma-thión

In Latin America and some parts of Spain, it is pronounced with an s sound instead:

es-ti-ma-sión

Both are normal depending on the variety of Spanish.

Not in any especially tricky way. A simple guide is:

im-pre-VIS-to

The stress is on vis.

A few pronunciation points:

• v and b are pronounced very similarly in Spanish
• the r in imprevisto is a Spanish tapped r
• the i and o are pure vowel sounds, not diphthongs like in some English words

So a rough pronunciation for an English speaker is:

eem-preh-BEES-toh or eem-preh-VEES-toh

The exact b/v sound will depend on position and accent, but you do not need to force an English-style v.

Yes. If you are talking about more than one unforeseen issue, you would make the noun and verb plural:

• Los imprevistos cambian la estimación. = The unforeseen issues change the estimate.
• Los imprevistos cambian las estimaciones. = The unforeseen issues change the estimates.

Notice the agreement:

• el imprevisto cambia
• los imprevistos cambian

That subject-verb agreement is very important in Spanish.