El primer fallo del informe era pequeño, pero el segundo cambió toda la frase.

Primer is the shortened form of primero. In Spanish, primero becomes primer when it comes directly before a singular masculine noun.

• el primer fallo = the first mistake
• el primero = the first one

So here, because fallo is a singular masculine noun and primer comes right before it, primer is required.

Here, fallo means mistake, fault, or error.

In Spain, fallo is quite common and can suggest:

• a mistake in a text
• a flaw in a system
• a fault in reasoning
• even a bug or defect

Error is also correct and often more neutral or general.

So:

• un fallo del informe = a mistake/fault in the report
• un error del informe = an error in the report

In this sentence, fallo sounds natural, especially in Spain.

Because de + el contracts to del in Spanish.

• de + el = del
• a + el = al

So:

• el primer fallo del informe = the first mistake of/from the report

This contraction is normally mandatory, except when el is part of a proper name:

• del libro
• but de El Escorial

Era is the imperfect form of ser, while fue is the preterite.

Here, era pequeño describes the mistake: it gives background information about what it was like. That is a typical use of the imperfect.

• era pequeño = it was small/minor

If you said fue pequeño, it would sound more like you are viewing the smallness as a completed fact or event, which is less natural in this context.

A useful contrast:

• era pequeño = description
• cambió = completed action that moved the story forward

Cambió is the third person singular preterite of cambiar.

It is used because the sentence refers to a completed action:

• the second mistake changed the whole sentence

This is not just background description; it is the main event.

So the contrast is very natural:

• era pequeño → background/description
• cambió → finished action

Spanish often omits a noun when it is already clear from context. This is called ellipsis.

So:

• el primer fallo ... pero el segundo ...

means:

• the first mistake ... but the second one ...

The noun fallo is understood, so it does not need to be repeated.

This is very common in both Spanish and English:

• I like the red shirt, not the blue one
• Prefiero el rojo, no el azul

Both are possible.

• toda la frase = the whole sentence / the entire phrase
• la frase entera = the entire sentence / phrase

In this sentence, toda la frase is a very natural way to say the whole sentence.

Also note agreement:

• toda matches frase, which is feminine singular

Examples:

• todo el informe = the whole report
• toda la frase = the whole sentence/phrase
• todos los errores = all the errors

It can mean either, depending on context, which can be confusing for English speakers.

In everyday Spanish, frase often refers to a string of words or a sentence-like unit. In grammar, however:

• frase can mean phrase
• oración more specifically means sentence

So if the English translation says sentence, that is fine here in context. But be aware that frase does not always map perfectly to English sentence.

In Spanish, most descriptive adjectives usually come after the noun.

So:

• un fallo pequeño = a small/minor mistake

That is the normal order.

If an adjective comes before the noun, it often adds a more subjective, stylistic, or emphatic nuance. For a straightforward description, post-noun position is the usual choice.

In your sentence:

• el primer fallo del informe era pequeño

pequeño is simply describing the size or importance of the mistake.

It can easily mean minor rather than physically small.

With something abstract like fallo, Spanish often uses size words metaphorically:

• un fallo pequeño = a small/minor mistake
• un gran error = a big/serious error

So in this sentence, pequeño most likely means:

• not very serious
• minor
• insignificant compared with the second one

Yes, Spanish often uses definite articles in places where English might not.

Here, El primer fallo means the first mistake. The article is needed because you are referring to a specific item in a sequence.

Spanish commonly uses articles with:

• ordinal numbers: el primero, el segundo
• general references that are definite in context
• many noun phrases where English might be more flexible

So Primer fallo del informe... without el would sound incomplete here.

The subject is understood from the noun phrases:

• El primer fallo del informe → subject of era
• el segundo (understood: fallo) → subject of cambió

So the structure is:

• El primer fallo del informe era pequeño
• pero el segundo cambió toda la frase

Spanish often leaves subjects unstated when they are already clear, but here the noun phrases themselves clearly act as the subjects.

It would change the meaning and tone.

• pero = but, showing contrast
• y = and, simply adding information

The sentence is contrasting the two mistakes:

• the first was minor
• the second had a major effect

So pero is the right choice because it highlights that contrast. Using y would weaken that idea:

• ..., y el segundo cambió toda la frase
  = ... and the second changed the whole sentence

That is grammatical, but less expressive than pero.