Cuptorul este încă cald.

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

• cuptor = oven
• cuptorul = the oven

Unlike English, Romanian usually puts the at the end of the noun instead of before it.

Cuptor is a neuter noun.

In Romanian, neuter nouns behave:

• like masculine in the singular
• like feminine in the plural

So in the singular you get:

• cuptorul = the oven
• cald = masculine/singular form of warm

In the plural, it changes:

• cuptoarele = the ovens
• calde = warm (plural)

So:

• Cuptorul este încă cald.
• Cuptoarele sunt încă calde.

Este means is, from the verb a fi = to be.

In a normal Romanian sentence like this, you do need the verb:

• Cuptorul este încă cald.

You can also use the very common short form:

• Cuptorul e încă cald.

But you would not normally leave it out completely.

Here încă means still, and it modifies the idea of being warm.

So:

• este încă cald = is still warm

This placement is very natural in Romanian.
You may also hear slightly different word orders, but this is the most neutral and standard one.

Not always. Încă can mean still, and in some contexts it can also correspond to yet.

In this sentence:

• încă = still

Examples:

• E încă aici. = He/She is still here.
• Nu e încă gata. = It isn’t ready yet.

So the exact English translation depends on the context.

Because cald is a predicate adjective after the verb to be.

After este, adjectives normally appear in their regular agreement form, without a definite article:

• cuptorul is singular
• in the singular, this neuter noun behaves like masculine
• so the adjective is cald

Compare:

• cuptor cald = warm oven
• cuptorul este cald = the oven is warm

You do not say caldul here, because the adjective is not being used as a definite noun.

It usually does not, because Romanian has a postposed definite article: the article is attached to the noun.

So instead of:

• the oven

Romanian says:

• cuptorul

This is one of the big structural differences between English and Romanian.

Yes. In everyday speech, Romanians very often use e instead of este:

• Cuptorul e încă cald.

This is completely normal and very common in conversation.
The version with este is also correct, and may sound a bit more careful or formal.

The standard, neutral order is:

• Cuptorul este încă cald.

Romanian word order is somewhat flexible, but changing it often changes the emphasis.

For example:

• Încă este cald cuptorul. — unusual, marked
• Cuptorul încă este cald. — possible, but less neutral than the original
• Cuptorul e încă cald. — very natural

So for a learner, the original order is the safest and most natural choice.

A rough pronunciation guide is:

koop-TOR-ul YES-teh UHN-kuh kald

A few notes:

• cuptorul has the stress on -tor-
• este is pronounced roughly YES-te
• î in încă is a Romanian sound with no exact English equivalent
• ă in încă is a short, relaxed vowel, similar to the a in sofa

A more natural spoken version is often:

• Cuptorul e încă cald.