Mi primo confunde el número decimal con el porcentaje cuando hace cálculos rápidos.

In modern Spanish, possessives like mi, tu, su, nuestro usually replace the article, so you say mi primo, not el mi primo.

• mi primo = my cousin
• el primo = the cousin

Using both together is not normal in standard modern Spanish. Older Spanish and some regional varieties may do things differently, but for everyday Spanish from Spain, mi primo is the correct form.

Yes. Primo is masculine, so it means male cousin. The feminine form is prima for female cousin.

• Mi primo = my male cousin
• Mi prima = my female cousin

Spanish often makes this distinction where English usually just says cousin.

The verb confundir is commonly used with con when you say that one thing is mistaken for another:

• confundir A con B = to confuse A with B

So:

• confunde el número decimal con el porcentaje
  = he confuses the decimal number with the percentage

This is the standard pattern. It works very much like English to confuse X with Y.

Spanish often uses the definite article more than English does. Here, el número decimal and el porcentaje refer to the two concepts being mixed up, so the article sounds natural.

• el número decimal = the decimal number / the decimal
• el porcentaje = the percentage

In English, you might sometimes leave out the article in a general statement, but Spanish often keeps it.

Here número decimal means decimal number. In context, it refers to a number written in decimal form, like 0,25 or 0.25 depending on notation.

Yes, in many contexts Spanish speakers might simply say el decimal if the meaning is obvious. But el número decimal is more explicit and clearer for learners.

A small Spain-specific note: in Spain, decimal notation usually uses a comma, so 0,25 is standard rather than 0.25.

Because the sentence describes a habitual or general situation, not something happening right now.

• cuando hace cálculos rápidos = when he does quick calculations
• cuando está haciendo cálculos rápidos = when he is doing quick calculations

The simple present (hace) is the normal choice for general truths, habits, or repeated situations. The progressive form (está haciendo) would sound more like a specific ongoing moment.

The verb hacer is very common and has a broad range of meanings, including to do and to make. In the expression hacer cálculos, it means to do calculations.

Some common examples:

• hacer ejercicio = to exercise
• hacer una pregunta = to ask a question
• hacer cálculos = to do calculations

So cuando hace cálculos rápidos literally means when he does quick calculations.

In Spanish, adjectives often come after the noun, especially when they describe a factual quality.

• cálculos rápidos = quick calculations

Also, rápidos must agree with cálculos:

• cálculos is masculine plural
• so the adjective is rápidos (masculine plural)

Compare:

• cálculo rápido = quick calculation
• cálculos rápidos = quick calculations

Usually, cálculos rápidos is the most natural wording here. Putting the adjective before the noun is possible in some cases in Spanish, but it often changes the tone or emphasis.

• cálculos rápidos = normal, descriptive
• rápidos cálculos = more literary or emphatic, less neutral

In everyday speech, cálculos rápidos is the better choice.

Because confundir here is being used as a normal transitive verb with a clear object:

• Mi primo confunde el número decimal con el porcentaje

The subject is mi primo, and the thing he confuses is el número decimal.

You can also see confundirse in Spanish, but it means something more like to get confused or to be mistaken:

• Mi primo se confunde = my cousin gets confused / makes a mistake

That version focuses more on his confusion in general. The original sentence focuses on exactly what he confuses with what.

Yes. Porcentaje is the standard, everyday word in Spain for percentage.

You may also encounter:

• tanto por ciento = percent / percentage

But porcentaje is extremely common and natural in modern Spanish.

Yes. Cuando can refer to different kinds of time relationships, and the verb form helps show the meaning.

In this sentence:

• cuando hace cálculos rápidos

it means when(ever) he does quick calculations, so it describes a repeated/general situation.

If you were talking about the future, Spanish often changes the verb form in the cuando clause:

• Cuando haga cálculos rápidos, tendrá que concentrarse más.
  = When he does quick calculations, he’ll have to concentrate more.

So cuando + present often gives a habitual meaning, while cuando + subjunctive often points to the future.

Yes, that is possible:

• Mi primo confunde el número decimal con el porcentaje al hacer cálculos rápidos.

This means something like My cousin confuses the decimal number with the percentage when doing quick calculations.

The difference is small:

• cuando hace cálculos rápidos sounds like a normal time clause: when he does quick calculations
• al hacer cálculos rápidos sounds a bit more like in the course of doing or when doing

Both are correct, but the original sentence is very natural and straightforward.

In Spain Spanish, confunde is pronounced roughly like:

• con-FOON-deh

The c in confunde is before o, so it has a hard k sound, like in cat.

A rough breakdown:

• con = like kon
• fun = foon
• de = deh

The stress is on fun: con-FUN-de.

The original order is the most neutral and natural:

• Mi primo confunde el número decimal con el porcentaje cuando hace cálculos rápidos.

But Spanish does allow some flexibility for emphasis. For example:

• Cuando hace cálculos rápidos, mi primo confunde el número decimal con el porcentaje.

This puts more emphasis on the situation when he does quick calculations.

So the word order is not completely fixed, but the original version is the best default structure for learners.