Este problema é comum.

Este means this.

In European Portuguese, the basic contrast is:

• este / esta / estes / estas = this / these near the speaker
• esse / essa / esses / essas = near the person being spoken to, or something just mentioned
• aquele / aquela / aqueles / aquelas = that / those farther away from both

So Este problema é comum means This problem is common.

In everyday speech, especially outside very careful distinctions, people do not always keep the three-way contrast as strictly as textbooks present it, but it is still important to learn the standard system.

Because problema is a masculine noun in Portuguese, even though it ends in -a.

That surprises many English speakers, because they often learn that nouns ending in -a are usually feminine. That is often true, but there are exceptions.

Some common masculine nouns ending in -a are:

• o problema
• o sistema
• o programa
• o tema

Since problema is masculine, the demonstrative must also be masculine:

• este problema
• not esta problema

Because este already identifies the noun.

In Portuguese, a demonstrative like este usually replaces the need for the definite article:

• este problema = this problem
• o problema = the problem

So you normally say:

• Este problema é comum.

not:

• O este problema é comum.

In standard modern Portuguese, combining the article with este in this kind of sentence is not normal.

The accent shows that é is the verb is from ser.

Compare:

• é = is
• e = and

So the accent is very important because it distinguishes two different words.

Examples:

• Este problema é comum. = This problem is common.
• Este problema e aquele... would start a different sentence meaning This problem and that one...

The accent also helps indicate pronunciation.

É is the 3rd person singular of the verb ser (to be).

The present tense of ser is:

• eu sou = I am
• tu és = you are
• ele/ela/você é = he/she/you is/are
• nós somos = we are
• vós sois = you are
• eles/elas/vocês são = they/you are

In this sentence, the subject is este problema, which is singular, so you use é.

Because comum is an adjective whose singular form is the same for masculine and feminine.

So you get:

• o problema comum = the common problem
• a situação comum = the common situation

But in the plural, it changes:

• problemas comuns
• situações comuns

So in your sentence:

• Este problema é comum.

comum agrees with a singular noun, and that singular form is simply comum.

It can mean both, depending on context.

Comum may mean:

• frequent / usual
• ordinary / not unusual
• shared / common to more than one thing in some contexts

In Este problema é comum, the most natural reading is usually:

• This problem is common
• meaning This problem happens often or This problem is not unusual

Context decides the exact nuance.

Normally, no. In this sentence, comum is not being used before the noun.

There are two different structures:

1. Este problema é comum.
   Here comum comes after the verb é because it is a predicate adjective:
   This problem is common.

2. um problema comum
   Here comum directly describes the noun:
   a common problem

So you could say:

• Este é um problema comum. = This is a common problem.

But comum problema is not the normal way to say it.

They are close in meaning, but not identical.

• Este problema é comum.
  = This problem is common.
  The sentence talks about the problem itself and says it is common.

• Este é um problema comum.
  = This is a common problem.
  The sentence identifies this as belonging to the category of common problems.

The second version is often slightly more classificatory, while the first is more direct.

A broad European Portuguese pronunciation is:

Este problema é comum.
/ˈeʃ.tɨ pɾuˈble.mɐ ɛ kuˈmũ/

A few useful points:

• este often sounds roughly like ESH-tuh
• problema has stress on ble
• é sounds like eh
• comum ends with a nasal sound, roughly koo-MOONG without a full final g

In natural European Portuguese, some unstressed vowels are reduced, so it may sound more compressed than a learner expects.

Sometimes yes, but the meaning changes slightly.

• este problema = this problem
• esse problema = that problem / that problem you mentioned / that problem near you

So if you say:

• Esse problema é comum.

you are usually referring to a problem that is less close to the speaker, or one already mentioned in the conversation.

For a sentence presented in isolation, este problema is the straightforward choice for this problem.

It is neutral and completely natural in both formal and informal Portuguese.

You could use it in:

• conversation
• writing
• class
• reports
• explanations

There is nothing especially formal, informal, or idiomatic about it. It is a very standard sentence pattern.