Si discipula mendum videt, illud delere et iterum scribere debet.

Because discipula is the subject of videt: she is the one doing the seeing.

• discipula = nominative singular, the schoolgirl / female student
• mendum = accusative singular, the direct object of videt, meaning the thing seen

So in discipula mendum videt, the pattern is:

• discipula = subject
• videt = verb
• mendum = object

Latin marks these roles with case endings, not mainly with word order.

Si means if and introduces a conditional clause.

So the sentence has two parts:

• Si discipula mendum videt = If the student sees a mistake
• illud delere et iterum scribere debet = she ought to erase it and write it again

This is a very common Latin pattern: si + verb, followed by the main clause.

Latin present tense often covers both ideas that English separates:

• she sees
• she is seeing

So videt can mean either, depending on context. In a general statement like this, English usually prefers sees.

Also, videt is:

• 3rd person singular
• present tense
• active voice
• indicative mood

So it means he/she/it sees.

Illud means that or it, and here it refers back to mendum.

Since mendum is:

• neuter
• singular
• and is the object of delere

the pronoun must match it in gender and number, and here it appears in the accusative singular neuter form:

• nominative neuter singular: illud
• accusative neuter singular: illud

So illud means it = the mistake.

Both id and illud can mean it, but they are not exactly the same.

• id is the neuter form of is, ea, id, often a more basic or weaker it / that
• illud is the neuter form of ille, illa, illud, often meaning that more distinctly

In many teaching sentences, illud is used clearly to point back to something already mentioned. It can feel a bit more emphatic or more clearly demonstrative than id.

So here, illud means something like that thing / it, referring to mendum.

Because they depend on debet.

Debet means ought, must, or should, and in Latin verbs like this are commonly followed by an infinitive:

• delere debet = she ought to erase
• scribere debet = she ought to write

So the structure is:

• debet = finite verb, she ought
• delere et scribere = infinitives explaining what she ought to do

This is very similar to English ought to erase and ought to write, except Latin uses the bare infinitive without a separate word like to in translation.

Because its subject is discipula, which is singular.

Debet is the 3rd person singular present of debere:

• debeo = I ought / I must
• debes = you ought
• debet = he/she/it ought

Since one student is being discussed, Latin uses the singular form debet.

Latin word order is more flexible than English because case endings show grammatical function.

Placing debet at the end is very natural Latin style. In fact, Latin often likes to put the main verb near the end, especially in straightforward prose.

So although English would usually say:

• she ought to erase it and write it again

Latin comfortably says:

• it to erase and again to write she ought

That does not sound strange in Latin.

Iterum means again, and here it goes with scribere:

• iterum scribere = to write again

So the idea is not erase again, but rather:

1. see the mistake
2. erase it
3. write it again

In other words, the corrected form is written a second time.

Because Latin verbs usually already contain the subject in their endings.

• videt = she/he/it sees
• debet = she/he/it ought

Since discipula has already been stated, Latin does not need to repeat a pronoun like ea for she. The subject is understood from both the context and the verb ending.

Yes, the object is understood from context.

The sentence does not explicitly repeat the object after scribere, but the meaning is something like:

• erase it and write it again
• or more naturally, erase it and rewrite it

The reader understands that what is written again is the corrected form of whatever contained the mistake. Latin often leaves an object unstated when it is obvious from context.

A learner might want to identify each word like this:

• si = if
• discipula, discipulae = female student, schoolgirl
• mendum, mendi = fault, mistake, error
• video, videre, vidi, visum = see
• ille, illa, illud = that
• deleo, delere, delevi, deletum = destroy, erase, delete
• et = and
• iterum = again
• scribo, scribere, scripsi, scriptum = write
• debeo, debere, debui, debitum = owe; ought, must

Looking at the dictionary forms helps explain why the forms in the sentence are what they are.