Si el resultado es decimal, la profesora nos deja redondearlo.

Because they are two different words:

• si = if
• sí = yes / himself, herself, itself in some contexts

So in Si el resultado es decimal..., si introduces a condition: if the result is decimal.

Spanish often uses the definite article (el, la, los, las) more than English does.

Here, el resultado means the result. Even if English might sometimes say just if the result is a decimal or if the answer is decimal, Spanish naturally uses el resultado.

So:

• el resultado = the result

This is completely normal Spanish style.

Here, decimal is being used like an adjective, not as a noun.

So:

• es decimal = is decimal / has a decimal part
• es un decimal would sound more like it is a decimal number

In this sentence, the idea is not really to classify the result as a decimal number in a technical way, but to describe it: if the result comes out decimal.

So es decimal is the more natural choice here.

In this context, decimal means that the result is not a whole number and includes digits after the decimal separator.

For example:

• 3 is not decimal here
• 3.5 is decimal
• 7.25 is decimal

In Spain, decimals are usually written with a comma instead of a point, so you might see:

• 3,5
• 7,25

But the word decimal is the same.

La profesora means the female teacher.

A few useful points:

• profesor = male teacher
• profesora = female teacher

So the sentence specifically refers to a woman.

Also, Spanish normally includes the article here:

• la profesora = the teacher

Even if English often says the teacher without thinking about gender, Spanish usually makes that distinction.

Nos deja means she lets us or she allows us.

It comes from the verb dejar, which can mean:

• to leave
• to let / allow

Here it clearly means to let / allow.

So:

• la profesora nos deja... = the teacher lets us...

Nos is the object pronoun meaning us.

In la profesora nos deja redondearlo, it shows who is being allowed to do the action:

• la profesora = the teacher
• nos deja = lets us
• redondearlo = round it

So the structure is:

• [subject] + [object pronoun] + dejar + infinitive

Example:

• Mi madre me deja salir. = My mother lets me go out.
• El profe nos deja usar calculadora. = The teacher lets us use a calculator.

Because the sentence is stating a general rule or habitual situation.

• Si el resultado es decimal, la profesora nos deja redondearlo.
• If the result is decimal, the teacher lets us round it.

This is a normal if + present, present structure used for things that are generally true, regularly happen, or are presented as rules.

It is not talking about one specific past event, but about what happens in that situation.

Redondearlo breaks down like this:

• redondear = to round
• lo = it

So:

• redondearlo = to round it

The lo refers back to el resultado.

Since resultado is masculine singular, the pronoun is lo.

Compare:

• el número → redondearlo
• la cifra → redondearla

In Spanish, object pronouns can attach to an infinitive.

So:

• redondearlo = to round it

This is very common after another conjugated verb, especially with verbs like poder, querer, deber, dejar, necesitar.

For example:

• Quiero verlo. = I want to see it/him.
• Podemos hacerlo. = We can do it.
• La profesora nos deja redondearlo. = The teacher lets us round it.

That said, Spanish often allows another placement too.

Yes. That is also grammatical.

You can say:

• La profesora nos deja redondearlo
• La profesora nos lo deja redondear

Both mean essentially the same thing: The teacher lets us round it.

The difference is just pronoun placement:

1. attached to the infinitive

   • redondearlo

2. before the conjugated verb

   • nos lo deja redondear

In this sentence, redondearlo is a very natural and straightforward option.

Because lo is a direct object pronoun, and it refers to el resultado.

What is being rounded?

• the result

So the result is the direct object:

• round it = redondearlo

Meanwhile, nos is the person being allowed to do the action:

• she lets us

So:

• nos = us
• lo = it

Using le here would not be standard for this meaning.

Because this is a normal real condition.

Spanish usually uses:

• si + present, then present/future/imperative

Here:

• Si el resultado es decimal...
• If the result is decimal...

This is a real or possible situation, so the indicative is used:

• es

Spanish does not normally use the subjunctive directly after si in this kind of sentence.

It is a real condition or general condition.

Structure:

• Si + present indicative, present indicative

So:

• Si el resultado es decimal, la profesora nos deja redondearlo.

This is used for:

• rules
• habits
• general truths
• repeated situations

Other examples:

• Si llueve, nos quedamos en casa. = If it rains, we stay home.
• Si tengo tiempo, te llamo. = If I have time, I call you / I’ll call you.

Yes, very common.

Dejar + infinitive means to let someone do something.

Pattern:

• dejar + indirect object + infinitive or
• with pronouns: dejar + pronoun + infinitive

Examples:

• Me deja entrar. = He/She lets me come in.
• Nos dejan salir antes. = They let us leave early.
• No me deja tocar eso. = He/She won’t let me touch that.

So in your sentence:

• nos deja redondearlo = lets us round it

Yes.

In maths, redondear means to round a number.

Examples:

• redondear a 2 decimales = to round to 2 decimal places
• redondear al alza = to round up
• redondear a la baja = to round down

So redondearlo here clearly means to round the result.

It changes for number, but not for masculine/feminine in the singular.

So you get:

• resultado decimal = masculine singular
• cifra decimal = feminine singular
• resultados decimales = masculine plural
• cifras decimales = feminine plural

The singular form is the same:

• decimal

The plural is:

• decimales

Yes, it is natural and correct.

A Spanish speaker might also say similar things such as:

• Si el resultado tiene decimales, la profesora nos deja redondearlo.
• Si sale decimal, la profesora nos deja redondearlo.

But the original sentence is perfectly good Spanish. It sounds like a classroom or maths-related statement.

The structure is:

• Si el resultado es decimal,
  conditional clause

• la profesora nos deja redondearlo.
  main clause

More specifically:

1. Si
2. el resultado
3. es
4. decimal
5. la profesora
6. nos
7. deja
8. redondearlo

So the second part is roughly:

• the teacher
  • us
    • lets
      • round it

This order is very natural in Spanish.