В этом письме есть одна странность.

Because here в means in, so it requires the prepositional case.

• в этом письме = in this letter
• в это письмо would mean into this letter, which does not fit this sentence

So:

• это письмо = this letter (dictionary/basic form)
• в этом письме = in this letter

A very common pattern is:

• в + prepositional for location: in/at
• в + accusative for motion into something

Examples:

• в доме = in the house
• в дом = into the house

Because это has to agree with the noun and also change for case.

Here the noun is письмо:

• singular
• neuter
• prepositional case after в

So этот becomes этом in this form:

• nominative: это письмо
• prepositional: в этом письме

You can think of этом as the form meaning in this / about this for masculine and neuter singular nouns in the prepositional case.

Because письмо is in the prepositional case after в.

The basic form is:

• письмо = letter

But after в meaning in, it changes:

• в письме = in the letter

So the ending changes from -о to -е.

This is a very common neuter pattern:

• окно → в окне
• море → в море
• письмо → в письме

Есть here means there is / there exists.

So the structure is:

• В этом письме есть... = In this letter there is...

This is the normal Russian way to say that something exists or is present somewhere.

Compare:

• На столе есть книга. = There is a book on the table.
• В комнате есть окно. = There is a window in the room.

In this sentence, есть does not mean eats. It is the verb to be / to exist in the sense of presence.

Because одна must agree with странность, and странность is a feminine noun.

Russian one changes by gender:

• один = masculine
• одна = feminine
• одно = neuter

Since the noun is:

• странность = feminine

you get:

• одна странность = one oddity / one strange thing

Compare:

• один стол = one table
• одна книга = one book
• одно письмо = one letter

Mostly from its form. Nouns ending in -ость are usually feminine.

Examples:

• новость = news
• скорость = speed
• возможность = possibility
• странность = oddity / strangeness

Also, the word одна confirms it, because adjectives and numerals agree with feminine nouns.

One thing that can confuse learners is the soft sign ь. A noun ending in ь can be masculine or feminine, so you often need to learn the gender with the word. But -ость nouns are very commonly feminine.

It can be literally one, but in sentences like this it often works a bit like English a certain or one for emphasis.

So одна странность can suggest:

• one oddity
• a certain odd thing
• one strange detail

It makes the sentence feel slightly more pointed than just saying есть странность.

Compare:

• В этом письме есть странность. = There is something strange about this letter.
• В этом письме есть одна странность. = There is one odd thing in/about this letter.

So одна may be counting, but it can also add a slight stylistic emphasis.

Because with есть in an existential sentence, the thing that exists is usually in the nominative case.

So:

• странность is the subject-like noun
• есть says that it exists
• в этом письме gives the location/context

This is the same pattern as:

• На столе есть книга.
• В городе есть музей.
• У меня есть вопрос.

In all of these, the thing that exists is in the nominative:

• книга
• музей
• вопрос
• странность

Usually, no—not if you want the clear meaning there is.

Russian often omits the present tense of to be in sentences like:

• Он студент. = He is a student.

But existential есть is different. It often stays when you mean there is / there exists / there is present.

So:

• В этом письме есть одна странность. = natural
• В этом письме одна странность. = sounds incomplete or changes the meaning/emphasis

Without есть, the sentence would usually not work as the same neutral statement of existence.

Yes, Russian word order is flexible, but the original order is the most neutral.

Neutral order:

• В этом письме есть одна странность.
• Literally: In this letter there is one oddity.

This presents:

1. the setting/location first: в этом письме
2. then the existence: есть
3. then the new information: одна странность

Other orders are possible, but they change emphasis. For example:

• Одна странность есть в этом письме.
  This is much less natural and sounds marked or literary.

• Есть одна странность в этом письме.
  Possible, but it emphasizes the existence first.

So the original sentence is a very normal information flow: where? → what is there?

It can mean several kinds of letter/message, depending on context.

Письмо can refer to:

• a traditional written letter
• a formal letter
• an email, in many modern contexts

So в этом письме could mean:

• in this letter
• in this email
• in this written message

Russian uses письмо quite broadly, just as English letter sometimes extends beyond paper in translation, though in modern usage email can still be expressed with электронное письмо if needed for clarity.

A standard pronunciation guide with stress is:

В э́том письме́ есть одна́ стра́нность.

Stress:

• э́том
• письме́
• одна́
• стра́нность

A rough English-style approximation:

• v EH-təm pees-MYE yest' ad-NA STRAHN-nast'

A few points:

• в is pronounced like v
• письме has a soft пь sound
• есть ends with a soft ть
• странность has a soft final ть as well

If you're reading naturally, the main intonational weight often falls near странность, since that is the new information.