Ex duabus crumenis haec plenissima est, sed illa minus argenti habet.

Literally, ex duabus crumenis means out of the two purses or of the two purses.

• ex means out of, from among
• duabus crumenis is in the ablative plural, because ex takes the ablative

Latin often puts this kind of phrase first to set the scene: we are choosing one item from a group of two.

Because duo is one of the Latin numerals that changes form for gender and case.

Here the noun is crumenis, from crumena, which is feminine. Also, it must be ablative plural after ex. So the correct form is:

• masculine/neuter dative or ablative plural: duobus
• feminine dative or ablative plural: duabus

So ex duabus crumenis is exactly what you would expect: from two purses.

Because haec and illa do not mean these and those here. They mean this one and that one.

The sentence first mentions the group: the two purses. Then it picks out each purse individually:

• haec = this purse
• illa = that purse

The noun crumena is understood but not repeated. This is very common in Latin.

They are feminine because the understood noun is crumena, and crumena is feminine.

So the grammar is really:

• haec crumena
• illa crumena

Latin often leaves the noun out when it is obvious from context.

Plenissima is the feminine singular nominative superlative of plenus, meaning full.

Its three degrees are:

• plenus = full
• plenior = fuller
• plenissimus / plenissima / plenissimum = fullest, very full

Here it agrees with the understood feminine singular noun crumena, so haec plenissima est means this one is fullest or this one is the fullest.

That is a very good question. In more classical usage, when comparing two things, Latin often prefers the comparative, so you might expect haec plenior est = this one is fuller.

However, learners do sometimes meet sentences like this with a superlative in a simple comparison. In context, plenissima est is being understood as the fullest / the fuller one.

So:

• more classical expectation with two items: plenior
• the sentence as given: plenissima, understood from context

A learner should at least be aware that plenior would be a very natural alternative.

Argenti is genitive singular.

It appears after minus, because Latin often uses minus + genitive to mean less of something. This is called a partitive genitive.

So:

• minus argenti = less silver / less money
• literally, something like less of silver

That is why Latin does not use argentum here.

Because minus here is the normal Latin way to express a smaller quantity of something.

With mass nouns such as silver, water, money, and similar words, Latin often uses:

• plus + genitive
• minus + genitive

So:

• plus aquae = more water
• minus pecuniae = less money
• minus argenti = less silver / less money

Using minor here would not be the normal idiom for quantity.

It literally means silver, but in many contexts it can also mean silver coin, cash, or money more generally.

So in this sentence, minus argenti habet most naturally means it has less money in it.

This is similar to how a material name can stand for wealth or currency.

Because its subject is illa, meaning that one, with crumena understood.

So the second clause is essentially:

• illa crumena minus argenti habet

Since that subject is singular, the verb is singular too:

• habet = has

Latin word order is much more flexible than English word order.

This sentence is arranged to highlight the comparison:

• Ex duabus crumenis sets up the group first
• haec points to one purse
• plenissima est gives the description
• sed illa shifts to the other purse
• minus argenti habet explains how it differs

English usually needs a more fixed order, but Latin can move words around as long as the endings make the grammar clear.