El error puede arruinar el plan.

Because error is a masculine noun in Spanish, so it takes el.

A useful pattern:

• Nouns ending in -or are very often masculine.
• So you get el error, el color, el motor.

The article has to agree with the noun’s gender:

• el error
• not la error

Puede is the third-person singular present tense of poder.

So it matches the subject el error, which is singular:

• yo puedo = I can
• tú puedes = you can
• él/ella/usted puede = he/she/you can
• el error puede = the error can

In this sentence, puede can mean something like:

• can
• may
• is able to

Depending on context, English might translate it slightly differently, but can is the most straightforward choice here.

Because after poder, the next verb normally stays in the infinitive.

This is the same structure as in English:

• can ruin
• puede arruinar

Not:

• can ruins
• puede arruina

So the pattern is:

• poder + infinitive
• puede arruinar
• puede ayudar
• puede cambiar

Arruinar means to ruin, to wreck, or sometimes to spoil, depending on context.

It usually suggests causing serious damage to something, either literally or figuratively.

Examples:

• arruinar el plan = ruin the plan
• arruinar la sorpresa = spoil the surprise
• arruinar un negocio = ruin a business

So in this sentence, arruinar is a strong verb: the error does not just affect the plan a little; it can seriously damage it.

Yes, error can often mean mistake in normal Spanish.

That is important because English sometimes uses error in a more technical or formal way, but Spanish error is very common in everyday language too.

For example:

• Cometí un error = I made a mistake
• Fue un error = It was a mistake

So even though the word looks like English error, it is often used where English would naturally say mistake.

Because el plan means the plan, referring to a specific plan.

Spanish uses articles very regularly, and in this sentence the speaker is talking about a particular plan, not just any plan.

Compare:

• el plan = the plan
• un plan = a plan

So:

• El error puede arruinar el plan = a specific error can ruin a specific plan
• Un error puede arruinar un plan = an error can ruin a plan, more general

Because el plan is a direct object that refers to a thing, not a person.

Spanish uses the personal a mainly with direct objects that are:

• people
• sometimes pets or personified beings

So:

• Veo a María = I see María
• Veo el plan = I see the plan

In your sentence, el plan is just a thing, so no a is needed.

No. This is a very standard subject + verb + object sentence:

• El error = subject
• puede arruinar = verb phrase
• el plan = direct object

That makes it a very natural beginner-friendly sentence structure.

Spanish can change word order for emphasis, but this version is the neutral, ordinary order.

The main thing to notice is the rr.

In Spanish, rr is a strong rolled/trilled r. So arruinar does not sound like English ruin with an English r.

A simple breakdown is:

• a-rrui-nar

Things to notice:

• rr = rolled/trilled sound
• the stress falls on the last syllable: -nar

So the stressed part is:

• a-rrui-NAR

If pronunciation is your focus, arruinar is a good word to practise because it combines the trilled rr with a common verb ending.

Puede is usually pronounced roughly like:

• PWEH-deh

Important points:

• The ue works together, so it sounds like pwe
• The stress is on the first syllable: PUE-de

So:

• PUE-de

This happens because poder is a stem-changing verb:

• poder
• puedo
• puedes
• puede

That o → ue change is very common in Spanish verbs.

Yes, it could, depending on context.

Puede often expresses:

• ability: can
• possibility: may / might

So this sentence could suggest:

• an error can ruin the plan
• an error might ruin the plan

Without more context, can is usually the safest basic translation, but might is not impossible if the speaker is emphasizing possibility rather than ability.

Only if the subject is already clear from context.

Spanish often drops subject pronouns like él or ella, but dropping a full noun like el error is different. If you remove it, the sentence no longer explicitly says what can ruin the plan.

• El error puede arruinar el plan = The error can ruin the plan
• Puede arruinar el plan = He/she/it can ruin the plan, or context-dependent

So if you want to be clear that the error is the subject, keep el error.

Yes. That changes the nuance.

• El error = the error, a specific error
• Un error = an error, any error / one error

So:

• El error puede arruinar el plan sounds like you are talking about a particular error already known in the conversation.
• Un error puede arruinar el plan sounds more general: any error could ruin the plan.

Both are correct. The difference is mainly about specific versus non-specific reference.