Per rimam una gutta aquae in mensam cadit.

The subject is una gutta aquae.

The main noun there is gutta = drop, and it is in the nominative singular, which is the normal case for the subject.

• una = one / a single, agreeing with gutta
• aquae = of water

So the whole subject means one drop of water.

Because per takes the accusative.

• per = through
• rimam is the accusative singular of rima = crack, fissure

So per rimam means through a crack.

This is a very common Latin pattern:

• per viam = through the road / along the road
• per urbem = through the city

Because in can take either the accusative or the ablative, depending on the meaning.

Here it takes the accusative because there is motion toward something:

• in mensam = onto the table / into the table's surface area

Compare:

• in mensam cadit = it falls onto the table
• in mensa est = it is on the table

So:

• in + accusative = movement into/onto
• in + ablative = position in/on

Because Latin uses the genitive to express of water.

• aqua = water
• aquae = of water

So gutta aquae literally means a drop of water.

This is a very normal Latin construction: noun + genitive.

Other similar examples:

• pars urbis = part of the city
• copia vini = a supply of wine

Una is agreeing with gutta and means one or a single.

Since gutta is:

• feminine
• singular
• nominative

the form must be una.

This is from the numeral/adjective unus, una, unum = one.

So una gutta can mean:

• one drop
• a single drop

Depending on context, it may sound a little more emphatic than just English a drop.

Because the sentence is not talking about water in general, but about one drop of water.

Latin often uses a noun plus a genitive to express this kind of idea:

• gutta aquae = a drop of water
• pars panis = a piece of bread
• multitudo hominum = a crowd of people

So aquae depends on gutta, not on the verb.

Because Latin word order is much more flexible than English word order, and it is very common for the verb to come at or near the end of the sentence.

So:

• Per rimam una gutta aquae in mensam cadit

is perfectly natural Latin.

But Latin could also rearrange the words without changing the basic meaning, for example:

• Una gutta aquae per rimam in mensam cadit
• In mensam per rimam una gutta aquae cadit

The endings, not the position alone, tell you how the words function.

Not very naturally.

A very literal order would be:

• Through a crack one drop of water onto the table falls.

That is understandable, but not normal English.

Latin allows this because the case endings show the relationships:

• rimam = object of per
• gutta = subject
• aquae = genitive
• mensam = object of in
• cadit = verb

So when reading Latin, it is usually better to focus on the forms and constructions rather than trying to force English word order onto it.

Cadit is:

• third person singular
• present tense
• active voice
• from cadere = to fall

So it means:

• he/she/it falls

Here the subject is una gutta aquae, which is singular, so cadit is singular too.

Because the subject is singular, not plural.

• una gutta = one drop
• therefore the verb must be singular: cadit

If the subject were plural, the verb would be plural too:

• guttae aquae cadunt = drops of water fall

In this sentence, it most naturally means onto the table.

Latin in + accusative can mean movement into or onto, depending on what makes sense with the noun.

With mensa = table, English usually says onto the table, because a drop falls onto the surface.

So although in often corresponds to English in, here the natural English meaning is onto.

It describes the path the drop takes.

The drop falls through a crack. So per rimam tells you the route or channel through which the action happens.

That is exactly the kind of meaning per + accusative often has in Latin: movement through something.

Yes. Latin has no articles equivalent to English the or a/an.

So:

• rima can mean a crack or the crack
• mensa can mean a table or the table
• gutta can mean a drop or the drop

The context decides which English article is best.

That is why una gutta can help add the sense one drop or a single drop, but Latin still does not have a true article.

It can do both.

In a sentence like this, una gutta may simply mean one drop, but it can also suggest a single drop.

So it may add a slight emphasis: not many drops, just one.

English often does the same thing:

• one drop of water
• a single drop of water

Here are the basic dictionary forms:

• per = through
• rima, rimae = crack, fissure
• unus, una, unum = one
• gutta, guttae = drop
• aqua, aquae = water
• in = in, into, onto
• mensa, mensae = table
• cado, cadere, cecidi, casum = fall

Knowing the dictionary form helps you recognize why the forms in the sentence look the way they do:

• rimam from rima
• aquae from aqua
• mensam from mensa
• cadit from cadere