Si quis tabellam suam perdit, magistra alteram ei dat, ne scribere desinat.

Because after si (if), Latin often uses quis/quid as an indefinite pronoun, where English would say someone, anyone, or whoever.

So:

• si quis... = if anyone...

This is a very common pattern in Latin. The same thing often happens after words like:

• si = if
• nisi = unless
• ne = lest / so that not
• num = in questions expecting no

So here quis does not mean who? It means anyone / someone.

Tabellam suam is in the accusative singular because it is the direct object of perdit (loses).

Breakdown:

• tabella = writing tablet
• tabellam = accusative singular
• suam = accusative singular feminine, agreeing with tabellam

So:

• tabellam suam perdit = loses his/her own tablet

The adjective suam has to match tabellam in:

• gender: feminine
• number: singular
• case: accusative

Suus, sua, suum is the reflexive possessive adjective, used when the possessor is the subject of the clause.

Here the subject of perdit is quis:

• Si quis tabellam suam perdit
  = If anyone loses his/her own tablet

So suam refers back to quis.

By contrast, eius would mean his/her referring to someone else, not the subject of that clause.

So:

• suam = his/her own
• eius = his/her, belonging to another person already mentioned

Alteram means another one or a second one.

It is feminine accusative singular because the noun tabellam is understood:

• magistra alteram ei dat
• literally: the teacher gives another [tablet] to him/her

Latin often leaves out a noun when it is obvious from context. So alteram is standing on its own, but it really means:

• alteram tabellam

A learner may wonder why not aliam. In many contexts:

• aliam = another, a different one
• alteram = the other of two / a second one

In classroom Latin, alteram often simply means another one.

Ei is dative singular.

It is the indirect object of dat:

• magistra alteram ei dat
• the teacher gives another one to him/her

So:

• alteram = the thing being given, direct object
• ei = the person receiving it, indirect object

This is very common with verbs of giving:

• do alicui aliquid = I give something to someone

Latin has no articles. That means it has no exact equivalent of the or a/an.

So magistra can mean:

• teacher
• the teacher
• a teacher

Which one you use in English depends on context. In this sentence, English naturally says the teacher.

Because ne scribere desinat is a purpose clause.

• ne = so that ... not, lest
• desinat = subjunctive, because purpose clauses use the subjunctive

So the phrase means:

• so that he/she does not stop writing
• or more naturally, so that the student can keep writing

This is one of the most important uses of the subjunctive in Latin:

• ut
  • subjunctive = positive purpose
• ne
  • subjunctive = negative purpose

So:

• magistra alteram ei dat, ne scribere desinat
• the teacher gives him/her another one so that he/she will not stop writing

Latin normally uses ne by itself for a negative purpose clause.

So:

• ut scribat = so that he/she may write
• ne scribat = so that he/she may not write

That means Latin does not usually say ut non for ordinary negative purpose.

Here:

• ne scribere desinat = so that he/she may not stop writing

That is the standard Latin way to express the idea.

Because desino often takes a complementary infinitive.

• desino scribere = I stop writing
• desinat scribere = he/she may stop writing

In your sentence the order is:

• scribere desinat

but the idea is the same as:

• desinat scribere

So the infinitive scribere completes the meaning of desinat.

The understood subject is the same person referred to earlier by quis and ei.

So the meaning is:

• If anyone loses his/her tablet, the teacher gives that person another one, so that he/she does not stop writing.

Latin often leaves a subject pronoun unstated when it is clear from context.

So although no separate word for he/she appears before desinat, Latin expects you to understand it.

Latin word order is much more flexible than English word order because Latin shows grammatical relationships mostly through endings, not position.

English depends heavily on word order:

• The teacher gives another one to him.

Latin can move words around more freely:

• magistra alteram ei dat
• ei magistra alteram dat
• alteram magistra ei dat

All of these could work, though some word orders sound more natural than others depending on emphasis.

In your sentence, the order is normal enough and keeps related ideas clear:

• condition first: Si quis...
• main action: magistra alteram ei dat
• purpose: ne scribere desinat

• perdit = present indicative
• dat = present indicative
• desinat = present subjunctive

The first two are present indicative because they state the general situation:

• If anyone loses...
• the teacher gives...

This is a general present condition, something that happens whenever the situation arises.

Then desinat is present subjunctive because it is inside a purpose clause introduced by ne.

So the sentence mixes:

• indicative for the main factual framework
• subjunctive for the intended result or purpose

It can be understood either way, but in most contexts if anyone is the best translation.

That is because quis after si is indefinite and general:

• si quis tabellam suam perdit
  = if anyone loses his/her tablet

English sometimes uses someone in similar sentences, but anyone better captures the general rule-like sense.

No. Tabella here means a writing tablet, not a dining table or desk.

In a school context, this is a small board or wax tablet used for writing. So:

• tabellam suam perdit = loses his/her writing tablet

That is why the teacher gives another one and why the purpose is ne scribere desinat — so the student can continue writing.