Magna severitas pueros interdum terret, sed severitas sine ira discipulos docere potest.

Because severitas is a feminine noun in Latin, and the adjective has to agree with it in gender, number, and case.

• severitas = feminine, singular, nominative
• so magnus must become magna

That is why Latin says magna severitas = great strictness or great severity.

Here severitas is in the nominative singular, because it is the subject of the verb.

In the first clause:

• Magna severitas = the subject
• terret = frightens
• pueros = the direct object

So the basic structure is:

• Magna severitas pueros terret
  = Great strictness frightens the boys

The second severitas is also nominative singular, because it is again the subject of potest.

Because they are the direct objects of the verbs.

• terret frightens whom? → pueros
• docere teach whom? → discipulos

Both pueros and discipulos are accusative plural:

• pueri → pueros
• discipuli → discipulos

A native English speaker often has to get used to this, because English mostly shows object relationships by word order, while Latin shows them by endings.

Because the verb agrees with the subject, not with the object.

Here:

• subject = magna severitas = singular
• object = pueros = plural

So Latin uses terret = it frightens, not a plural verb.

This is exactly like English:

• Strictness frightens the boys

  not

• Strictness frighten the boys

Interdum means sometimes or from time to time.

It modifies the verb terret, so:

• Magna severitas pueros interdum terret = Great strictness sometimes frightens the boys

Latin word order is flexible, so interdum can often move around without changing the basic meaning. Its position here is natural, but not the only possible one.

Latin often repeats a noun for clarity or balance, especially in a contrast like this:

• Magna severitas ... terret
• sed severitas sine ira ... docere potest

The repetition helps emphasize the contrast:

• great strictness may frighten boys,
• but strictness without anger can teach students.

Latin could sometimes omit repeated words in other contexts, but here repeating severitas makes the sentence neat and clear.

Sine means without, and it takes the ablative case.

So:

• ira is ablative singular of ira, irae = anger
• sine ira = without anger

This is a very common Latin pattern:

• cum amico = with a friend
• sine timore = without fear
• sine ira = without anger

For English speakers, this is worth memorizing as a rule: sine + ablative.

Because potest means is able or can, and Latin regularly uses an infinitive with verbs of being able, wanting, beginning, and so on.

So:

• potest docere = can teach

More literally:

• is able to teach

Here:

• severitas sine ira discipulos docere potest = strictness without anger can teach students

This is similar to English, where can is followed by a bare verb: can teach.

Because docere commonly takes a direct object in the accusative for the person being taught.

So:

• discipulos docere = to teach the students

Latin doceo can also sometimes take two accusatives, especially when both the person and the thing taught are stated, for example:

• magister pueros grammaticam docet = the teacher teaches the boys grammar

In your sentence, only the people being taught are mentioned, so discipulos is the accusative object.

Yes. They overlap in meaning, but they are not exactly the same.

• pueros = boys
• discipulos = students/pupils

The sentence seems to make a small contrast:

• severe strictness may frighten them as children
• but strictness without anger can teach them as students

So the author may be suggesting two different ways of looking at the same young people: as boys who feel fear, and as pupils who can be educated.

Not exactly. The first clause says magna severitas = great strictness, which sounds more intense. The second clause just says severitas sine ira = strictness without anger.

That difference matters:

• great / very strong strictness may frighten
• strictness without anger may teach

So the sentence is not simply repeating the same idea. It is contrasting harsh intensity with controlled discipline.

A helpful way to read it is to group the words by function:

• Magna severitas = subject
• pueros = object
• interdum = adverb
• terret = verb

Then:

• sed = but

And in the second clause:

• severitas sine ira = subject phrase
• discipulos = object
• docere = infinitive
• potest = main verb

So a very literal layout would be:

• Great strictness the boys sometimes frightens, but strictness without anger the students to teach can.

That sounds strange in English, but it is perfectly normal in Latin because the endings show the relationships.

A learner often wants to know what forms to look up in a dictionary. Here are the main ones:

• magnus, magna, magnum = great, large
• severitas, severitatis = strictness, severity
• puer, pueri = boy
• interdum = sometimes
• terreo, terrere, terrui, territus = frighten
• sed = but
• sine = without
• ira, irae = anger
• discipulus, discipuli = student, pupil
• doceo, docere, docui, doctus = teach
• possum, posse, potui = be able, can

Knowing the dictionary form helps you see why the endings in the sentence look the way they do.