Sudul este cald.

Because -ul is the Romanian definite article attached to the end of the noun.

• sud = south
• sudul = the south

Romanian usually puts the at the end of the noun instead of using a separate word like English does.

It can refer to the southern part of a place or the South as a region, depending on context.

So Sudul este cald can mean something like:

• The south is warm
• The southern region is warm

If the larger context is missing, Romanian allows this same form for both ideas.

Este is the 3rd person singular of the verb a fi = to be.

So:

• eu sunt = I am
• tu ești = you are
• el/ea este = he/she/it is

Since sudul is a singular subject, Romanian uses este.

In standard Romanian, no.
You normally need the verb a fi in sentences like this.

So:

• Sudul este cald. = correct
• Sudul cald. = not standard Romanian

In casual speech, people often shorten este to e:

• Sudul e cald.

That is very common and natural.

Because cald agrees with the noun sudul, and sud is a neuter noun.

A useful rule for Romanian neuter nouns is:

• in the singular, they behave like masculine
• in the plural, they behave like feminine

Since this sentence is singular, the adjective takes the masculine singular form:

• cald = masculine/neuter singular
• caldă = feminine singular

So sudul este cald is correct.

Because here cald is a predicate adjective after the verb to be, so it does not take a definite article.

Compare:

• Sudul este cald. = The south is warm.
• sudul cald = the warm south / the warm southern region

In the sentence you gave, cald is describing the subject through the verb este, not directly as part of the noun phrase.

The normal, neutral word order is:

• Sudul este cald.

You can change the order for emphasis or style, but it sounds less neutral:

• Cald este sudul. = more literary or contrastive
• Este cald sudul. = generally unnatural in this context

So for learners, Sudul este cald is the best basic pattern.

A simple English-friendly approximation is:

• SOO-dool YES-teh kahld

A more careful breakdown:

• su as in soon without the final n
• dul with a clear dool
• este = YES-te
• cald has a short a, and the l is pronounced before d

Romanian spelling is fairly phonetic, so pronunciation usually follows the written form closely.

Yes. It means the same thing.

• Sudul este cald. = more formal or neutral
• Sudul e cald. = more common in everyday speech

Both are correct.

Yes, but it would usually change the meaning or the structure.

For example:

• Sudul este cald. = The south is warm.
• În sud este cald. = In the south, it is warm.
• La sud este cald. = To the south, it is warm / In the south, it is warm depending on context

Without -ul, sud often appears in a prepositional phrase such as în sud or la sud, rather than as the subject by itself.