Hoc opus hodie faciendum est.

Hoc means this.

More exactly, it is the neuter singular form of hic, haec, hoc. It matches opus, which is also neuter singular, so hoc opus means this work or this task.

Because opus is a neuter noun.

The demonstrative hic, haec, hoc has to agree with the noun it modifies in gender, number, and case. Since opus is neuter singular, the correct form is hoc.

So:

• hic = masculine singular
• haec = feminine singular
• hoc = neuter singular

It is nominative singular.

In this sentence, hoc opus is the grammatical subject of faciendum est. Even though English often says This work must be done today, where work feels like the thing receiving the action, Latin treats it as the subject in this kind of passive construction.

Also, both hoc and opus have the same form in the nominative and accusative neuter singular, so context tells you the case here.

Here opus means work, task, or piece of work.

It is a regular noun here, not the special expression opus est meaning there is need of. So in this sentence, opus is simply the thing that must be done.

Faciendum est is a gerundive construction meaning must be done, has to be done, or is to be done.

• faciendum is the gerundive of facere (to do, make)
• est means is

Together, gerundive + esse often expresses necessity or obligation.

So hoc opus faciendum est literally means something like:

this work is needing-to-be-done

Natural English: this work must be done.

Because the gerundive must agree with the noun it goes with.

Here the noun is opus, which is:

• neuter
• singular
• nominative

So the gerundive is also:

• neuter
• singular
• nominative

That gives faciendum.

If the noun were feminine singular, you would expect facienda. If masculine singular, faciendus.

Correct: the sentence does not explicitly say who must do it.

In Latin, with this construction, the person who has the obligation can be shown by the dative of agent:

• Mihi hoc opus hodie faciendum est = I must do this work today
• Nobis hoc opus hodie faciendum est = We must do this work today

Without a dative, the sentence just states the necessity in a general way: This work must be done today.

Hodie means today.

It tells you when the work must be done. It is an adverb, so it modifies the whole idea of must be done, not just the noun opus.

So:

• hoc opus = this work
• hodie = today
• faciendum est = must be done

Yes. Latin word order is much more flexible than English word order.

Hoc opus hodie faciendum est is perfectly natural, but you could also see things like:

• Hodie hoc opus faciendum est
• Hoc hodie opus faciendum est
• Faciendum est hoc opus hodie

The basic meaning stays the same because the endings show how the words fit together. Different word orders can change the emphasis a little.

For example:

• Hodie first may emphasize today
• faciendum est first may emphasize the necessity

It is grammatically passive in form, but it expresses an obligation that may feel active in meaning.

English often switches between:

• This work must be done
  and
• Someone must do this work

Latin uses the passive-looking gerundive construction here, so the thing that needs doing becomes the subject:

• hoc opus ... faciendum est

So the form is passive, but the idea is about what needs to happen.

Its basic force is usually necessity: must be done, has to be done, needs to be done.

In some contexts, English may translate it a little more softly as should be done, but the Latin construction itself normally suggests real obligation or necessity rather than a mild suggestion.

So if you are learning the pattern, must be done is the safest starting meaning.

No.

Faciendum est does not simply mean will be done. It means must be done or has to be done. The sentence expresses obligation, not just future time.

If you said hoc opus hodie fiet, that would mean this work will be done today.
But hoc opus hodie faciendum est means this work needs to be done today.

A very natural Latin way is:

Mihi hoc opus hodie faciendum est.

Literally, that is To me this work must be done today, but in idiomatic English it means:

I must do this work today.

That is a very common feature of the gerundive of obligation: the person responsible is put in the dative, not usually in a normal active subject position.