Tredecim nummi in crumena manent, sed tredecim mala emere non possumus.

Because tredecim is an indeclinable numeral in Latin. That means it does not change its form for case, gender, or number.

So Latin uses:

• tredecim nummi = thirteen coins
• tredecim mala = thirteen apples

Even though nummi and mala are in different cases, tredecim stays tredecim.

Because nummi is the subject of manent.

• nummi = nominative plural, the coins
• manent = remain / are left

So Tredecim nummi manent means thirteen coins remain.

If Latin had nummos, that would be accusative plural, which would usually make coins the direct object of a verb instead.

Because malum meaning apple is a neuter noun.

For neuter nouns in the plural:

• nominative plural = mala
• accusative plural = mala

So here mala is the accusative plural direct object of emere:

• tredecim mala emere = to buy thirteen apples

It is not malas, because malas would be feminine accusative plural, and apple is not feminine here.

Yes, here it is best understood as the noun mala = apples.

That is clear from the context:

• emere means to buy
• buying apples makes sense naturally
• the sentence contrasts coins with things you want to buy

So although mala can also be a form of the adjective malus, -a, -um meaning bad, here it is the noun apples.

Because in takes the ablative when it means in or inside a place without motion.

• in crumenā = in the purse
• this shows location

Latin uses:

• in + ablative for location: in the purse
• in + accusative for motion toward/into: into the purse

So in crumena manent means remain in the purse, not go into the purse.

Because its subject is plural: tredecim nummi.

Latin verbs agree with their subject in person and number:

• manet = he/she/it remains
• manent = they remain

Since thirteen coins is plural, Latin uses manent.

Manent means remain, stay, or are left. It gives a more specific idea than sunt.

So:

• Tredecim nummi sunt in crumena would mean There are thirteen coins in the purse
• Tredecim nummi in crumena manent means Thirteen coins remain in the purse or There are thirteen coins left in the purse

So manent emphasizes that this is what is still left.

Because after possumus Latin normally uses a complementary infinitive.

• possumus = we are able / we can
• emere = to buy

Together:

• emere possumus = we can buy
• emere non possumus = we cannot buy

This is very normal Latin structure: possum + infinitive.

Because non usually goes with the verb it negates.

Here:

• non possumus = we cannot

So tredecim mala emere non possumus means we cannot buy thirteen apples.

This is slightly different from something like possumus non emere, which would mean we can choose not to buy. In your sentence, the point is lack of ability, not a choice not to act.

Because the sentence means we cannot buy thirteen apples.

The ending -mus shows we:

• possum = I can
• potes = you can
• potest = he/she/it can
• possumus = we can

Latin often leaves out the pronoun nos because the verb ending already shows who is meant.

Yes. Latin word order is much freer than English word order because the endings show each word’s role.

This sentence is arranged to make the contrast clear:

• Tredecim nummi ...
• sed tredecim mala ...

Repeating tredecim in both halves highlights the problem: the number of coins is the same as the number of apples, but that still is not enough to buy them.

So the order is not random; it helps emphasize the contrast.