Mi hermano no entiende una ecuación larga si no ve primero la fórmula.

In Spanish, a possessive adjective like mi normally goes directly before the noun:

• mi hermano = my brother
• tu casa = your house
• su libro = his/her/their book

You do not say el mi hermano in standard modern Spanish.

Hermano mío is possible, but it is a different structure and usually sounds more emphatic, emotional, literary, or contrastive:

• Mi hermano = neutral, normal
• Hermano mío = more like my brother! / that brother of mine

So Mi hermano is the natural everyday choice here.

In Spanish, basic negation is formed by putting no directly before the conjugated verb:

• entiende = he understands
• no entiende = he does not understand

So:

• Mi hermano no entiende... = My brother doesn’t understand...

This is the normal way to make a sentence negative in Spanish.

This is a very common question because si no and sino are different.

si no = if ... not

It introduces a condition:

• si no ve primero la fórmula = if he doesn’t see the formula first

Here, si means if, and no negates ve.

sino = but rather / except

This is used to correct or replace something:

• No es largo, sino corto. = It isn’t long, but rather short.

So in your sentence, it must be si no because the meaning is conditional:
He doesn’t understand a long equation if he doesn’t first see the formula.

Because after si meaning if, Spanish normally uses the indicative, not the subjunctive, for real or possible conditions in the present/future.

So:

• si ve = if he sees
• si no ve = if he doesn’t see

Using vea here would sound wrong in standard Spanish for this kind of condition.

A useful rule:

• si + present indicative for normal present/future conditions
  
  • Si estudia, aprende. = If he studies, he learns.
• si + imperfect subjunctive for unlikely/hypothetical conditions
  
  • Si viera la fórmula, entendería la ecuación. = If he saw the formula, he would understand the equation.

So ve is correct because this is a normal present condition.

Spanish often leaves out subject pronouns when the verb ending already shows who the subject is.

Here:

• entiende = he/she understands
• ve = he/she sees

Because the subject is already clear from the context, Spanish does not need to repeat mi hermano or add él.

So this is natural:

• Mi hermano no entiende... si no ve...

You could say si él no ve primero la fórmula, but that would usually add emphasis or contrast, for example if you want to stress he and not someone else.

This is about the choice between the indefinite article and the definite article.

una ecuación larga

This presents it as a long equation, not necessarily one specific equation already identified for the listener.

la fórmula

This suggests the formula is specific, known, or relevant in context.

So the sentence feels like:

• he doesn’t understand a long equation
• unless he first sees the formula

In other words, the equation is being mentioned more generally, while the formula is treated as something specific or expected in the situation.

Of course, article choice can change with context. For example:

• Mi hermano no entiende la ecuación larga... would mean a specific long equation.

In Spanish, adjectives usually come after the noun:

• una ecuación larga = a long equation
• un libro interesante = an interesting book
• una casa grande = a big house

That is the normal position.

Putting the adjective before the noun is sometimes possible, but it often changes the tone or emphasis, and in many cases it sounds less neutral.

So:

• una ecuación larga = the standard, natural order

Because the personal a is mainly used with human direct objects, especially specific ones.

Compare:

• Veo la fórmula. = I see the formula.
• Veo a mi hermano. = I see my brother.

In your sentence, una ecuación larga is a thing, not a person, so there is no personal a:

• entiende una ecuación larga

That is completely normal.

Primero here is an adverb meaning first. Spanish adverbs are often flexible in position, but the placement used here is very natural:

• si no ve primero la fórmula

This means if he doesn’t see the formula first.

You may also hear slight variations, depending on style and emphasis, such as:

• si no ve la fórmula primero

Both are understandable. The version in your sentence is perfectly normal and sounds natural.

Because this is a full sentence with a subject, so the verb must be conjugated.

The subject is mi hermano, which is third person singular, so the verbs are:

• entiende from entender
• ve from ver

Compare:

• entender = to understand

• entiende = he/she understands

• ver = to see
• ve = he/she sees

So the sentence needs the conjugated forms because it is saying what my brother does.

Yes. Ver is irregular in some forms, and ve is the third person singular present indicative:

• yo veo
• tú ves
• él/ella ve
• nosotros vemos
• vosotros veis
• ellos ven

So in this sentence:

• si no ve primero la fórmula = if he doesn’t see the formula first

It may look short, but it is the correct standard form.

Yes, comprende is possible, because comprender can also mean to understand.

However, entender is extremely common in everyday Spanish and sounds very natural here:

• Mi hermano no entiende...

In many contexts, entender and comprender are very close in meaning. Sometimes comprender can sound a bit more formal or slightly more like fully grasp, but the difference is often small.

So for this sentence, entiende is a very normal and natural choice.

Yes, Spanish word order has some flexibility, although the original sentence is very natural.

For example, these are also possible:

• Si no ve primero la fórmula, mi hermano no entiende una ecuación larga.
• Mi hermano, si no ve primero la fórmula, no entiende una ecuación larga.

These versions keep the same basic meaning, but the original order is simple and straightforward.

So the given sentence is a normal, neutral way to say it.