Clepsydra tempus metitur, dummodo aqua fluat.

Because metitur comes from metior, metīrī, mensus sum, a deponent verb.

Deponent verbs:

• have passive-looking forms
• but active meanings

So:

• metitur = he/she/it measures
• not is measured

Here the subject is clepsydra, so clepsydra metitur means the water clock measures.

They are in these cases:

• clepsydra = nominative singular
  It is the subject of metitur.

• tempus = accusative singular
  It is the direct object: the thing being measured.

• aqua = nominative singular
  It is the subject of fluat.

So the structure is:

• Clepsydra = the water clock
• tempus = time
• metitur = measures

and then:

• aqua = water
• fluat = flows

Because it is the direct object of metitur.

The verb metior takes an object in the accusative when it means measure. So:

• tempus metitur = it measures time

A native English speaker may expect something more like measures out time, but in Latin the simple accusative works naturally.

Here dummodo means provided that, so long as, or as long as.

It introduces a proviso or condition:

• Clepsydra tempus metitur, dummodo aqua fluat.
• A water clock measures time, provided that the water flows.

So the idea is not just a plain fact about flowing water; it is a condition under which the main statement is true.

Because dummodo regularly takes the subjunctive when it means provided that.

So:

• fluit = flows (indicative)
• fluat = may flow / should flow / be flowing in a dependent, conditional sense

In smoother English, we usually do not translate the subjunctive directly. We just say:

• provided that the water flows

But Latin shows that conditional/proviso idea by using dummodo + subjunctive.

Fluat is present subjunctive, third person singular, from fluo, fluere.

It is present because the sentence expresses a general present condition:

• a water clock measures time
• as long as / provided that water is flowing

If the sentence were talking about a different time frame, Latin might use a different tense. But here the present subjunctive fits the general, ongoing idea.

Because dummodo and si are not quite the same.

• si = if
• dummodo = provided that / as long as

Si gives a more neutral condition.
Dummodo emphasizes a requirement or limitation.

So this sentence is not merely saying if water flows, the clock measures time. It is saying something closer to:

• the clock measures time, but only on the condition that water is flowing

That is why dummodo is a good choice here.

Yes, the word order is perfectly normal.

Latin word order is much freer than English word order because the endings show the grammatical roles. So both of these are understandable:

• Clepsydra tempus metitur
• Clepsydra metitur tempus

The version with tempus before metitur may give a slight feeling of emphasis or simply sound stylistically good. Latin often places the verb near the end of the clause.

So the order here is natural Latin, even though it does not match standard English order exactly.

Because Latin has no articles.

So a noun like clepsydra can mean:

• a water clock
• the water clock

and aqua can mean:

• water
• the water

The exact choice in English depends on context. In a general statement like this, English often uses:

• A water clock measures time, provided that water flows.

But Latin itself does not mark that distinction with an article.

Clepsydra is a first-declension feminine noun, nominative singular here.

It is also a word borrowed from Greek, but in Latin it behaves like a regular first-declension noun in many forms. In this sentence, you only need to recognize that:

• clepsydra = subject
• therefore nominative singular

That is why the verb metitur is also singular.

Not here, at least not as the best reading.

Although parts of dum-type expressions can sometimes relate to time, dummodo in classical Latin commonly means provided that or so long as, and it normally introduces a proviso, not a simple time clause.

So in this sentence the most natural understanding is:

• provided that the water flows

rather than:

• while the water flows

The subjunctive fluat also supports that proviso meaning.

They are:

• metitur → metior, metīrī, mensus sum
  = measure

• fluat → fluo, fluere, fluxi, fluxum
  = flow

This is useful because metitur can confuse learners at first: if you know the dictionary form is metior, you can recognize it as a deponent verb and understand why it has passive endings but active meaning.