Quam diu suber movetur, amicus harundinem tenet.

Here quam diu is an idiomatic time expression meaning as long as or while.

A useful detail: in other contexts, especially in direct questions, quam diu? can mean how long? But in this sentence it introduces a time clause.

You may also see it written as one word: quamdiu.

No. In this expression, quam diu works together as a fixed adverbial phrase. Quam is not describing or agreeing with suber, amicus, or harundinem.

So you should understand quam diu as a unit, not as quam doing its usual adjective/pronoun job.

Movetur is the 3rd person singular present passive indicative of moveo.

Latin often uses a passive form where English would naturally use an intransitive active verb. So:

• suber movetur = the cork is moved
• but in normal English this often comes out simply as the cork moves or the cork is moving

The idea is that the cork is undergoing motion, rather than actively causing motion.

By contrast, movet would usually mean he/she/it moves something.

Here suber is nominative singular and is the subject of movetur.

A small complication: because suber is neuter, its nominative and accusative singular can look the same. But in this sentence it is clearly functioning as the subject, because:

• movetur needs a subject
• suber fits that role naturally
• there is no better accusative function for it in the clause

So the first clause is built around suber movetur = the cork moves / is moving.

Because it is the direct object of tenet.

• tenet = holds
• the thing being held is harundinem

So Latin puts harundinem in the accusative singular.

Its dictionary form is harundo, and the accusative singular of that noun is harundinem.

Because amicus is the subject of tenet in the main clause.

So the sentence has two clauses:

• quam diu suber movetur
• amicus harundinem tenet

In the second clause, amicus is the one doing the holding, so it appears in the nominative.

They are:

• suber, suberis — cork, neuter
• amicus, amici — friend, masculine
• harundo, harundinis — reed, feminine

This is useful because the sentence shows some inflected forms, not just dictionary forms:

• suber is already in its basic nominative singular form
• amicus is also nominative singular
• harundinem is the accusative singular form of harundo

Because Latin has no articles.

That means a noun like amicus can mean:

• a friend
• the friend

and context tells you which is meant.

The same is true for suber and harundinem. Latin leaves that distinction unstated unless the context makes it clear.

Yes, the word order is flexible.

Latin relies much more on case endings and verb forms than on rigid word order. So the grammar is shown mainly by:

• amicus being nominative
• harundinem being accusative
• movetur and tenet being finite verbs

This sentence puts the time clause first, which is very natural:

• quam diu suber movetur, then
• amicus harundinem tenet

A different order could still make sense, but the emphasis or style would change.

Both are present indicative, 3rd person singular:

• movetur = present passive indicative
• tenet = present active indicative

The Latin present can describe:

• something happening right now
• a repeated action
• a general situation

So this sentence could describe a scene in progress, or it could describe a regular pattern, depending on context.