Это существо умеет летать.

Because существо is a neuter singular noun, and the demonstrative this has to match it in gender and number.

• masculine: этот
• feminine: эта
• neuter: это
• plural: эти

So:

• этот человек = this person/man
• эта птица = this bird
• это существо = this creature

Even though это can also mean this is / it is in other sentences, here it is simply the neuter form of this.

In Russian, grammatical gender does not always match real-life sex.

Существо is grammatically neuter because of the type of noun it is and its ending, not because the creature is sexless. Russian often treats words for beings or objects according to grammar first.

So even if the creature is biologically male or female, the noun существо itself is still neuter, and adjectives/verbs in the past tense would agree with that grammatical gender.

It is in the nominative case, because it is the subject of the sentence.

The sentence structure is basically:

• Это существо = this creature
• умеет летать = knows how to fly / can fly

So существо is the thing doing the action, which is why it stays in the nominative.

This is a very common question, because both can sometimes translate as can in English.

• уметь = to know how to do something, to have a learned skill
• мочь = to be able to, to be capable of, to have the possibility

So:

• Это существо умеет летать = This creature knows how to fly / has the ability as a skill
• Это существо может летать = This creature can fly / is able to fly

In many contexts, both are possible, but умеет emphasizes that flying is a skill or natural ability the creature possesses.

Because after уметь, Russian normally uses an infinitive.

Pattern:

• уметь + infinitive

Examples:

• умеет читать = knows how to read
• умеет плавать = knows how to swim
• умеет летать = knows how to fly

If you said летит, that would mean is flying right now, which is a different idea.

Compare:

• Существо умеет летать = The creature can fly.
• Существо летит = The creature is flying.

Because летать is the imperfective infinitive, and that is the normal choice when talking about a general ability.

After уметь, Russian usually uses an imperfective verb, because the meaning is not about one completed action, but about being able to do something in general.

• уметь летать = to know how to fly / be able to fly

A perfective verb like полететь would mean something more like to set off flying or to start flying, which does not fit the idea of a general skill.

Умеет is the 3rd person singular present-tense form of уметь.

That means it agrees with a singular subject like существо.

Conjugation of уметь:

• я умею = I know how
• ты умеешь = you know how
• он / она / оно умеет = he / she / it knows how
• мы умеем = we know how
• вы умеете = you know how
• они умеют = they know how

So существо умеет means the creature knows how.

Yes, Russian word order is fairly flexible, though some versions sound more natural than others.

The neutral word order here is:

• Это существо умеет летать.

You could also say:

• Существо умеет летать. = The creature can fly.
• Летать это существо умеет. = As for flying, this creature can do it.
  This sounds more emphatic or contrastive.

So word order can change for emphasis, but the original version is the most straightforward.

The stress is:

• э́то
• существо́
• уме́ет
• лета́ть

A rough pronunciation guide:

• Э́то существо́ уме́ет лета́ть
• approximately: EH-ta su-shchis-TVÓ oo-MYE-yet li-TÁT’

A few notes:

• что / щ in существо is pronounced like a soft long shch
• ть at the end of летать is soft
• unstressed о often sounds closer to a in natural speech

Not normally. The most natural reading is simply:

• Это существо умеет летать = This creature can fly.

If you wanted to say This is a creature that can fly, Russian would more likely use a different structure, for example:

• Это существо, которое умеет летать.

So in your sentence, это существо is best understood as one noun phrase: this creature.