Этот гений известен.

Because этот has to agree with гений in gender, number, and case.

• гений is masculine
• it is singular
• here it is in the nominative case because it is the subject

So the correct form is этот.

Compare:

• этот гений — this genius
• эта книга — this book
• это окно — this window
• эти люди — these people

It is in the nominative singular.

In Этот гений известен, гений is the subject of the sentence, and subjects are normally in the nominative case.

Its dictionary form is also гений. Some other forms are:

• nominative: гений
• genitive: гения
• dative: гению
• instrumental: гением
• prepositional: о гении

So in this sentence, you are seeing the basic subject form.

In Russian, not all masculine nouns end in a hard consonant. Some masculine nouns end in -й or -ий.

So гений is simply one type of masculine noun. Its ending does not make it feminine or neuter.

Other masculine nouns with similar endings include:

• герой — hero
• музей — museum
• санаторий — sanatorium

So гений is masculine even though it ends in -ий.

Because известен is the short form of the adjective известный, and short-form adjectives are often used when the adjective is the main predicate of the sentence.

Here the structure is basically:

• Этот гений известен — This genius is famous / known

But when the adjective comes directly after the subject and means is ..., Russian often uses the short form.

Compare:

• известный гений — a famous genius
  
  • here известный modifies a noun directly
• гений известен — the genius is famous / known
  
  • here известен is the predicate

So известен is the natural form in this sentence.

Because in Russian, the verb to be is normally omitted in the present tense.

So:

• Этот гений известен literally looks like This genius famous/known
• but it means This genius is famous / known

In other tenses, Russian does use forms of to be:

• Этот гений был известен — This genius was famous
• Этот гений будет известен — This genius will be famous

So the missing is is completely normal.

It can mean either, depending on context.

Very often известен means:

• well-known
• famous
• known

In a sentence like Этот гений известен, English will often translate it as This genius is famous or This genius is well known.

But известен can also be used in broader ways, for example:

• Он известен всем — He is known to everyone
• Он известен своими работами — He is known for his works
• Он известен как учёный — He is known as a scientist

So the exact English wording depends on context, but the core idea is being known.

Yes. You could say:

• Гений известен.

That would still be grammatical.

The difference is that этот makes the noun more specific: this genius, meaning a particular one already identified in the situation or conversation.

Without этот, the sentence is more general and depends more heavily on context. It could mean something like:

• The genius is famous
• A genius is famous
• This genius is famous, if context already makes it clear

So этот helps point to a specific person.

Yes. Russian word order is more flexible than English word order.

The neutral order here is:

• Этот гений известен.

But other orders are possible for emphasis or style, such as:

• Известен этот гений.
• Гений этот известен.

These versions do not usually change the basic meaning, but they change the focus or emphasis.

As a learner, the safest neutral version is Этот гений известен.

The stress is:

• Э́тот ге́ний изве́стен.

A rough pronunciation guide:

• э́тот — EH-tət
• ге́ний — GYE-nee-y
• изве́стен — eez-VYE-styen

A few useful notes:

• гений has a soft ге
• the ий at the end sounds like a short -iy
• известен is stressed on ве

If you want to sound natural, make sure the stressed syllables are clear: Э́тот ге́ний изве́стен.

Yes. The short-form adjective must agree with the noun.

Forms of известен:

• masculine: известен
• feminine: известна
• neuter: известно
• plural: известны

Examples:

• Этот гений известен.
• Эта писательница известна.
• Это имя известно.
• Эти люди известны.

So in your sentence, известен is masculine singular because гений is masculine singular.