Cum tectum et fenestra refecta erunt, nullae stillae per rimam cadent.

Because here cum introduces a whole clause: cum tectum et fenestra refecta erunt.

The preposition cum meaning with is used with an ablative noun or pronoun, as in:

• cum amico = with a friend
• mecum = with me

But in your sentence, cum is followed by a subject and a verb, so it is the conjunction when.

refecta erunt is the future perfect passive.

It is built from:

• refecta = the perfect passive participle of reficio
• erunt = they will be

Together, they mean will have been repaired.

So:

• tectum et fenestra refecta erunt = the roof and window will have been repaired

This is a very common Latin pattern:

• perfect passive participle + form of sum

For reference, the verb is:

• reficio, reficere, refeci, refectum = repair, restore, fix

Because the repairing happens before the main future action.

The sentence means:

1. first, the roof and window get repaired
2. after that, no drops will fall

Latin often uses the future perfect in a time clause to show that one future action will be completed before another future action.

So:

• cum ... refecta erunt = when ... have been repaired
• cadent = will fall

English often does something similar, though not always in the same way. We usually say:

• When the roof and window have been repaired, no drops will fall
• or sometimes When the roof and window are repaired...

Latin is being very precise about the order of events.

Because it refers to two things together: tectum and fenestra.

The two nouns are:

• tectum = neuter singular
• fenestra = feminine singular

When Latin has multiple nouns of different genders, especially inanimate things, it often uses the neuter plural for an adjective or participle referring to them together.

So refecta is neuter plural: repaired things.

If both nouns were feminine, you would expect refectae instead.

Here tectum is nominative.

For many neuter nouns in Latin, the nominative and accusative singular have the same form. So tectum could be either in another sentence.

But here the context shows it is nominative because:

• it is joined to fenestra
• together they are the subject of refecta erunt
• the construction is passive, so they are the things being repaired

So the structure is:

• tectum et fenestra ... erunt = the roof and window ... will be

Because cadent is the main future result, not an earlier completed action.

• refecta erunt = an action completed first
• cadent = what will happen after that

So Latin uses:

• future perfect for the earlier future action
• simple future for the later future action

That contrast helps show the timeline clearly.

It means no drops or not any drops.

• stillae = drops, drips
• nullae = no, feminine plural to match stillae

So nullae stillae is literally no drops.

Latin often uses nullus, nulla, nullum as an adjective meaning no. That is more natural here than trying to use non with the noun.

rimam is accusative singular because the preposition per takes the accusative.

• per = through
• rima = crack, fissure, slit
• per rimam = through the crack / through a crack

Since Latin has no word for the or a, English can translate it either way depending on context.

Because this is a straightforward temporal use of cum: when.

The clause is simply stating the time at which the main action will happen. In that kind of sentence, Latin commonly uses the indicative, especially with a future-perfect idea like this one.

The subjunctive after cum is common in other uses, such as:

• since
• although
• background or circumstantial when

But here the meaning is just a plain future-time when, so the indicative fits.

No. Latin word order is much more flexible than English word order.

A very literal rearrangement would sound awkward in English. What matters most are the endings and the grammar:

• cum introduces the time clause
• tectum et fenestra are the things being repaired
• refecta erunt is the future perfect passive
• nullae stillae is the subject of the main clause
• per rimam tells where the drops would fall
• cadent is the main future verb

So good English translation should sound natural, even if the order changes.