Questo è il problema centrale.

Because problema is treated as a masculine singular noun in Italian, even though it ends in -a.

So the demonstrative has to match it:

• questo problema = this problem
• questa situazione = this situation

A lot of Greek-origin nouns in -ma are masculine in Italian, such as:

• il problema
• il sistema
• il programma
• il tema

Italian nouns do not always follow the simple pattern -o = masculine and -a = feminine. That pattern is common, but there are important exceptions.

Problema is one of those exceptions: it is masculine, so it takes:

• il problema = the problem

Its plural is also masculine:

• i problemi = the problems

So in this sentence, both questo and il agree with the masculine noun problema.

The accent is necessary because è is the verb essere in the he/she/it is form, and also you are in the formal Lei form.

• è = is

The accent distinguishes it from e, which means and.

Compare:

• Questo è il problema centrale. = This is the central problem.
• pane e acqua = bread and water

So the accent is not optional here.

È is the third-person singular present tense of essere (to be).

The full verb is:

• io sono = I am
• tu sei = you are
• lui/lei è = he/she is

In Questo è il problema centrale, questo is functioning like this or this one, and è links it to il problema centrale.

In Italian, articles are used more often than in English. Here, il is natural because the phrase is identifying something as the central problem, not just central problem in a vague sense.

So:

• il problema centrale = the central problem

If you remove il, the sentence sounds incomplete or unnatural in standard Italian.

In Italian, adjectives often come after the noun, especially when they are descriptive and factual.

So:

• problema centrale = central problem

This is the normal order here.

Italian adjective position is flexible, but moving an adjective before the noun can sometimes change the tone or emphasis. In this case, il problema centrale is the standard and most natural phrasing.

Some Italian adjectives have the same singular form for both masculine and feminine.

Centrale is one of them:

• il problema centrale = the central problem
• la questione centrale = the central question

Its plural changes:

• i problemi centrali
• le questioni centrali

So centrale is an adjective with:

• singular: centrale
• plural: centrali

In this sentence, questo is acting as a pronoun.

Why? Because it stands alone:

• Questo è il problema centrale. = This is the central problem.

It is not directly attached to a noun. If it were an adjective, it would modify a noun directly:

• Questo problema è centrale. = This problem is central.

So:

• questo alone = pronoun
• questo + noun = adjective/determiner

Yes, but it means something slightly different.

• Questo è il problema centrale = This is the central problem
  → it identifies it as the main or key problem

• Questo è un problema centrale = This is a central problem
  → it says it is one important central problem, but not necessarily the one main problem

So il and un change the meaning, just as the and a do in English.

Questo is pronounced roughly like KWEH-stoh.

A few helpful points:

• qu in Italian usually sounds like kw
• e here is a clear short vowel, not like English ee
• stress is on the first syllable: QUES-to

So: KWEHS-to

A natural pronunciation is roughly:

KWEHS-to eh eel pro-BLEH-ma chen-TRAH-le

A few notes:

• è is a short open eh
• il is usually light and quick
• problema has stress on ble
• centrale has stress on tra
• Italian vowels are usually pronounced clearly, without being reduced as much as in English

The plural would be:

Questi sono i problemi centrali.
= These are the central problems.

Here is what changes:

• questo → questi
• è → sono
• il problema → i problemi
• centrale → centrali

So all the agreeing words change to match the plural.

Yes. In Italian, questo can work in a broad way that covers both ideas depending on context.

For example:

• Questo è importante. = This is important.
• Questo è il problema centrale. = This is the central problem.

English sometimes adds one, but Italian often does not need a separate word for that.

Yes, but the nuance is a little different.

• centrale suggests something central, core, or at the heart of the issue
• principale suggests something main or principal

Often they are similar, but not always identical.

So:

• il problema centrale = the core/central problem
• il problema principale = the main problem

Depending on context, one may sound better than the other.