Haec sententia non est murmuranda, sed clare recitanda.

Because haec is agreeing with sententia, which is singular feminine nominative.

The forms are:

• haec sententia = this sentence
• hae sententiae = these sentences

So even though haec can sometimes mean these, here the noun it modifies is singular, so the meaning is this.

Because haec sententia is the subject of the sentence, so it must be in the nominative case.

The basic structure is:

• Haec sententia = subject
• non est murmuranda = is not to be mumbled
• sed clare recitanda = but to be recited clearly

If it were a direct object, then you would expect hanc sententiam instead.

This is the passive periphrastic, a very common Latin construction.

It is formed with:

• a gerundive: murmuranda, recitanda
• a form of sum: est

This construction usually expresses necessity, obligation, or what ought to be done.

So:

• est murmuranda = must be mumbled, is to be mumbled, or should be mumbled
• est recitanda = must be recited, is to be recited, or should be recited

Here the negative and contrast give:

• non est murmuranda, sed clare recitanda
• it should not be mumbled, but recited clearly

Because they are gerundives agreeing with sententia.

Since sententia is:

• feminine
• singular
• nominative

the gerundives must also be:

• feminine
• singular
• nominative

That gives:

• murmuranda
• recitanda

If the noun were masculine singular, you would get -andus; if neuter singular, -andum.

It is simply understood. Latin often leaves out a repeated word when the meaning is obvious.

So:

• Haec sententia non est murmuranda, sed clare recitanda

means the same as the fuller version:

• Haec sententia non est murmuranda, sed clare recitanda est

The second est is omitted because it would be repetitive.

Because clare is an adverb, while clara would be an adjective.

Here the idea is recited clearly, so we need an adverb modifying the verbal idea in recitanda.

• clare = clearly
• clara = clear, bright, famous depending on context

So:

• clare recitanda = to be recited clearly

not

• clara recitanda, which would not express the intended meaning.

Sed means but, and it sets up a contrast with non:

• non ... sed ... = not ... but ...

So the sentence is contrasting two ways of saying the sentence:

• not mumbled
• but clearly recited

This is a very common Latin pattern.

In this sentence, murmuranda comes from murmurare, to murmur. In natural English, mumbled is often the best way to express the idea.

So the literal idea is something like:

• not to be murmured

but in smoother English:

• not to be mumbled

A learner should notice that Latin vocabulary often overlaps with several English words, and the best translation depends on context.

Yes. The passive sense comes from the gerundive + est construction.

The sentence is not saying:

• someone should mumble this sentence

in an active form.

It is saying:

• this sentence should not be mumbled
• this sentence should be recited clearly

So sententia is the thing receiving the action.

Latin often leaves the agent unstated when it is general or obvious.

This sentence gives a general instruction:

• This sentence should not be mumbled, but recited clearly.

It does not need to say by you or by the reader, because that is understood from context.

If Latin wanted to name the person responsible, it could do so, often with a dative of agent in this construction, for example:

• discipulo haec sententia clare recitanda est
• the student must recite this sentence clearly

But in your sentence, the instruction is deliberately general.

Latin word order is flexible, but this order is very natural and clear.

It puts:

1. the subject first: Haec sententia
2. the first idea next: non est murmuranda
3. the contrasting idea after sed: clare recitanda

This arrangement highlights the contrast neatly:

• not mumbled
• but clearly recited

So the word order is doing rhetorical work, even though the grammar would still be understandable in a different order.