Magistra monet usum saepe plus valere quam timorem, si quis artem aliquam discere velit.

The main subject is magistra and the main verb is monet.

• magistra = the teacher (nominative singular, feminine)
• monet = advises / reminds / warns / points out

So the sentence begins with the basic core:

Magistra monet = The teacher advises / reminds

Everything after that gives the content of what she is saying.

After monet, Latin uses an indirect statement construction here: accusative + infinitive.

That is:

• usum = accusative subject of the infinitive
• valere = infinitive

So:

magistra monet usum ... valere
literally = the teacher advises/reminds that practice ... is stronger / counts for more ...

This is very common in Latin after verbs of saying, thinking, knowing, perceiving, and sometimes verbs like moneo in the sense used here.

Because it is the subject of the infinitive valere inside the indirect statement.

In English, we say:

• The teacher says that practice is more effective

In Latin, instead of that practice is, you often get:

• practice in the accusative
• is as an infinitive

So:

• usus valet = practice is effective / practice is stronger
• but in indirect statement:
• usum valere = that practice is effective

Also, usum is the accusative singular of the 4th-declension noun usus.

It comes from usus, which can mean use, experience, practice, or habitual exercise, depending on context.

Here, because the sentence is about learning a skill, practice is the most natural sense.

So usum here is not just use in the everyday English sense. It means something more like:

• practice
• practical experience
• actual repeated doing

That fits the idea of learning an art or skill.

Because it is being compared with usum after quam.

The structure is:

usum ... plus valere quam timorem

= practice to be worth more than fear

Since usum is accusative, the noun after quam is also in the corresponding case:

• usum ... quam timorem

This is normal in Latin comparisons. English does not show case clearly here, but Latin does.

plus valere is an idiomatic Latin expression meaning something like:

• to be stronger
• to count for more
• to have greater force
• to be more effective

So plus valere quam timorem means that practice has more force than fear or is more effective than fear.

As for plus:

• plus is the comparative form connected with multum
• it can be used adverbially or in a neuter substantive way to mean more

With verbs like valere, Latin very naturally says plus valere.

So this is not a noun agreeing with usum. It is part of the expression meaning to avail more / to count for more.

saepe modifies valere.

So the sense is:

• usum saepe plus valere
  = practice often counts for more

It does not mean that the teacher often warns, and it does not directly modify timorem.

A good way to hear the structure is:

The teacher advises that practice often is worth more than fear...

This is a very common Latin pattern.

After words like:

• si = if
• nisi = unless
• num
• ne

Latin often uses quis/quid instead of aliquis/aliquid.

So:

• si quis = if anyone / if someone
• not usually si aliquis

Here quis is indefinite, not interrogative. It does not mean who?

So si quis artem aliquam discere velit means:

• if anyone should want to learn some skill
• or more naturally, if anyone wants to learn a skill

Because the si-clause is part of the thought being reported after monet.

The main verb monet introduces indirect statement, and subordinate clauses inside reported speech are often put into the subjunctive in Latin.

So a direct version might be something like:

Usus saepe plus valet quam timor, si quis artem aliquam discere vult.

But when that idea is reported after monet, Latin can shift the subordinate clause into the subjunctive:

... si quis artem aliquam discere velit.

So velit here is best understood as part of the reported idea, not as an independent subjunctive floating by itself.

Because it depends on velit.

Latin often uses an infinitive after verbs of wanting, being able, beginning, and similar ideas, just as English does.

So:

• velit discere = wants to learn / may wish to learn

This is completely normal:

• volo discere = I want to learn
• potest facere = he can do
• coepit currere = he began to run

So discere is a complementary infinitive with velit.

artem aliquam is accusative singular, because it is the direct object of discere:

• discere artem = to learn a skill / art

About the word order:

• artem aliquam
• aliquam artem

Both are possible. Latin word order is flexible.

Here aliquam means some or a certain, and in context it can be understood naturally as some skill or any skill.

Putting aliquam after the noun is not strange in Latin. It may simply sound natural, or give a slight rhythm/emphasis to artem first.

A helpful way to unpack it is:

Magistra monet
[usum saepe plus valere quam timorem]
[si quis artem aliquam discere velit].

That is:

1. Magistra monet = The teacher advises/reminds
2. usum saepe plus valere quam timorem = that practice often counts for more than fear
3. si quis artem aliquam discere velit = if anyone wants to learn a skill

So the sentence is built in layers:

• main clause
• indirect statement
• conditional clause inside that thought

That layered structure is very typical of Latin prose.