Die City ist heute voll.

Because City is a borrowed noun in German, and in this meaning it is usually treated as feminine: die City.

So you get:

• die City = nominative
• der City = genitive/dative
• die City = accusative

This grammatical gender does not come from English, since English nouns do not have grammatical gender. It is simply how German uses the word.

Not quite. In German, die City often means the city center, downtown, or the main shopping/business area, rather than the whole city.

So Die City ist heute voll usually means something like:

• Downtown is crowded today
• The city center is full today

If you wanted to talk about the whole city, die Stadt is often more natural.

Yes, you could, but the meaning shifts a little.

• Die City ist heute voll = the downtown / central area is crowded
• Die Stadt ist heute voll = the city is crowded/full

In many contexts, die Stadt sounds more general and more standard, while die City can sound a bit more urban, modern, or like you are specifically talking about the center.

Here voll means full or, more naturally in English, crowded.

Literally:

• voll = full

But with places, German often uses voll where English would prefer crowded:

• Der Zug ist voll = The train is full / crowded
• Die Bar ist voll = The bar is full / crowded
• Die City ist heute voll = Downtown is crowded today

So the exact English translation depends on context.

Because German main clauses normally follow the V2 rule: the finite verb comes in the second position.

In this sentence:

• Die City = first element
• ist = finite verb, so it must come second
• heute voll = the rest

Structure:

• Die City | ist | heute voll.

If you move heute to the front, the verb still stays second:

• Heute ist die City voll.

That is a very important German word-order rule.

Yes, absolutely. That is also correct.

Compare:

• Die City ist heute voll.
• Heute ist die City voll.

Both mean the same thing, but the emphasis is slightly different:

• Die City ist heute voll focuses first on the city center
• Heute ist die City voll focuses first on today

This kind of fronting is very common in German.

Because German does not need a dummy it in this sentence. The real subject is already present:

• Die City = subject
• ist = is
• heute voll = crowded today

So German simply says:

• Die City ist heute voll.

English sometimes uses it in sentences like It is cold, but that is a different structure. Here German names the thing directly.

Yes. Voll is an adjective.

In this sentence it comes after the verb sein, so it is being used predicatively:

• Die City ist voll.

When adjectives are used this way in German, they do not take adjective endings.

Compare:

• Die City ist voll. → predicative, no ending
• eine volle City → attributive, with an ending

Because voll comes after ist and describes the subject through the verb sein. In that position, German adjectives usually stay in their basic form.

So:

• Die City ist voll. ✔
• Die City ist volle. ✘

You only add adjective endings when the adjective comes before a noun:

• eine volle U-Bahn
• eine volle Innenstadt

No. Heute is flexible.

All of these are possible, depending on style and emphasis:

• Die City ist heute voll.
• Heute ist die City voll.
• Die City ist voll heute. (possible, but less neutral in many contexts)

The most natural versions are usually:

• Die City ist heute voll.
• Heute ist die City voll.

German lets you move time expressions around, but the verb in a main clause still stays in second position.

Yes, it can be natural, especially in informal speech or in contexts where City clearly means downtown or the central shopping area.

That said, many speakers might more often say:

• Die Innenstadt ist heute voll.
• In der Stadt ist heute viel los.
• Die Stadt ist heute ziemlich voll.

So Die City ist heute voll is understandable and usable, but depending on region and style, another phrase may sound even more typical.

A few very natural alternatives are:

• Die Innenstadt ist heute voll.
  = The city center is crowded today.

• In der Innenstadt ist heute viel los.
  = There’s a lot going on downtown today.

• In der Stadt ist heute viel los.
  = The city is really busy today.

• Die Stadt ist heute ziemlich voll.
  = The city is pretty crowded today.

So the original sentence is fine, but learners should know that German often uses expressions like viel los when talking about a place being busy or lively.

You would usually use nicht:

• Die City ist heute nicht voll.
  = Downtown is not crowded today.

Here nicht comes before the adjective voll.

If you wanted to say not the city center, you would negate a different part:

• Nicht die City ist heute voll, sondern der Park.
  = It’s not downtown that is crowded today, but the park.

So the position of nicht depends on what you want to negate.

For a yes/no question, put the verb first:

• Ist die City heute voll?
  = Is downtown crowded today?

This is standard German question word order:

• statement: Die City ist heute voll.
• question: Ist die City heute voll?

You could also ask for more detail:

• Warum ist die City heute voll?
  = Why is downtown crowded today?
• Ist die City immer so voll?
  = Is downtown always this crowded?